Let's start with a device lots of people have seen: A CPAP.

A CPAP is "constant positive airway pressure" ventilation assistance device.

It helps people breath in - but it doesn't do much to pull air out of you. You exhale by relaxing the muscles in your chest (including the diaphragm under your lungs). Atmospheric air pressure pushes in on your chest wall and pushes air out of your lungs.

People who have severe breathing problems often need help moving air both in and out of their lungs. This can be especially true of people of people with emphysema, where air becomes "trapped" in larger spaces in your lungs. These larger spaces result from a breakdown in the membrane of the alveolar sacs in your lungs, which creates larger spaces from which you cannot exhale all the air. Air becomes trapped in these larger spaces, and you cannot get fresh, oxygen-rich air into the lungs. Pulling the air out with negative pressure can help these patients.

A BiPAP or ventilator can move air both into your lungs, and also pull air out of your lungs. The BiPAP and vent can also be set up to control how fast the inspiration and expiration happen - the usual ratio is 1:3 - ie, if it takes 1 second to inhale, then the machines will make you exhale over 3 seconds. There are many settings for both a BiPAP and ventilator - how much air to push in, at what pressure, for how long (or how many breaths per minute), how fast the air is pushed in, how long the air is kept in there, at what pressure, etc. The ventilator is hooked up to the oxygen supply on the wall of the hospital room (as is the BiPAP) and you can set the "fraction of inspired oxygen" (in medical lingo, "FiO2") to between 21% (the O2 concentration of room air) to 100%. As a patient's breathing deteriorates, the FiO2 is often increased. Breathing pure oxygen for long periods of time isn't recommended, and can cause other issues. 

One of the issues that a BiPAP and ventilator can control is "how much pressure needs to be left in the patient's lungs to keep the alveoli open?" In some situations (and COVID-19 can be one of those situations), the disease process attacks the ability of your lungs to make a "surfactant" - something that reduces the surface cohesion of fluids. In effect, a surfactant is sort of like a soap, but a surfactant makes the liquid (whether it is water or something else) "more slippery" or "more wet" without introducing the cleaning and grease-busting properties of soap. Lots of people here at NAT understand surfactant as something you add to water in a spray tank to make the "water wetter" and make pesticides sprayed on crops cover the leaves more evenly.

OK, so where am I going with this? In the lungs, surfactant affects the ability of your alveoli to re-open when they've completely closed after exhaling. If you lose your ability to produce surfactant in your lungs, your alveoli could close, and not re-open when pressured air is put back into your lungs.

If a doctor determines that a patient's lungs are having an issue how well the alveoli re-open, the doctor might call for the ventilator to have "PEEP" pressure raised. This is the pressure left in your lungs after you finish exhaling. Something like this can be accomplished only by a BiPAP or vent, because you need to have a good seal between the machine and your lungs to hold some pressure in the lungs at the end of exhaling. And it's not a lot of pressure - we're talking maybe 10+ cm of water column pressure.

Ok, rather than confuse people any more with a BiPAP, let's talk about a ventilator specifically. The big difference between a vent and a BiPAP or CPAP is that the latter two devices use a mask to access your airway. The mask is strapped to your face, and covers your nose and mouth. 

Now let's talk specifically about a ventilator. First, a ventilator will be connected to a ET (endotracheal) tube, which is previously inserted into your windpipe. There is a little "balloon" on the ET that is inflated with a syringe filled with air to create a seal that allows the vent to push pressure into your lungs and create negative pressure to help you exhale, as well as to prevent aspiration of saliva or stomach contents. 

Here's a picture of a ET tube:

http://breathinstephen.com/wp-content/uploads/2015/12/Endotracheal-Tube.jpg

OK, the "balloon" I wrote of above is on the left. The thin line with the blue widget on it - that's what is connected to a empty syringe that is filled with air and used to inflate and deflate the balloon on the left of the tube. The "ID 7.5 OD 10.0" is the ID/OD in millimeters This tube is about the size that would be used for many adults. They come in different diameters. The little arrows with numbers like 22, 24, 26, 28? Those are reference marks that the RT's and RN's will look at to make sure the tube isn't moving in the patient's throat/trachea when looking in on the patient regularly. The depth of the tube (those numbers of 22 to 28 or so) will be recorded in the patient's chart every time the tube is assessed. 

The connection to the air source is over on the right side. A ambu-bag (ie, a manual way of inflating the lungs) could be attached, or a ventilator. 

To intubate a patient, a doctor, respiratory therapist or paramedic will whip out a neat little tool called a laryngoscope, sometimes called a "Mac" or "Miller" blade. It's effectively a flashlight with a very specifically shaped blade to allow the MD/RT/medic to move the patient's vocal cords out of the way, and then slide in the ET tube without the bladder inflated. Once the ET is in properly, then a syringe is hooked up to the bladder fitting, then air is pushed in until it "feels" right, and the laryngoscope Blade is removed.

Here's a pic of laryngoscopes:

https://classconnection.s3.amazonaws.com/1865/flashcards/669379/jpg/macintosh-blade-miller-blade.jpg

Here's what they're looking for when they slide that laryngoscope into a patient's throat:

https://qph.fs.quoracdn.net/main-qimg-ee5eb10ca05c8b1061d89c8b90d555d4

Let's not get caught up on how they get the ET in there. Let's just assume that it is, then it gets tested for function with a bag. 

Because most people will fight the ET tube and ventilator when they're awake, most patients on a vent are put under with anesthesia. Sometimes, the anesthesia is a drug that doesn't relieve any pain, it simply knocks you out, like propofol (aka "Diprovan"). The reason why the patient needs to be anesthetized is that most patients will fight to remove the ET tube down their throat. Often, there is also a "NG" or "naso-gastric" or "OG" ("orogastric") tube is also put into a patient, to keep the stomach empty, to decrease the risk for aspiration of stomach contents.  The NG/OG tube is hooked up to the suction flow on the wall of the hospital room. Aspiration of stomach contents, saliva, etc is a Very Bad Thing in patients who are having breathing problems. If a patient aspirates something from their stomach, if they weren't on a ventilator before aspiration, they probably will be after aspiration. The aspirated material will often cause pneumonia all by itself, with no other disease/virus.

A CPAP or BiPAP machine, some people will fight, but they can get used to it. However, there's very little way to get used to dealing with a tube down your throat into your trachea. Your gag reflex (which is almost completely involuntary) commands your brain to get whatever is in your trachea out of there - now. Doesn't matter if it is a piece of food that "went down the wrong way" or the ET tube that's saving your life.  Sometimes, when patients are being brought out of anesthesia to remove the vent and the NG/OG tube, the MD's will have the nurses restrain the patient's arms to the bed, to prevent the patient from tearing the ET out of their throat while they're only semi-conscious.

ET tubes aren't a permanent or even a long term way to access a patient's airway. At some point, if the patient needs to be intubated for a long time (and MD's make the call on what a "long time" is), then a MD/surgeon will make a tracheostomy into the trachea through the front of the neck, and they will access the airway through that. But that's another story, so let's not get into that right now.

A patient who is on a ventilator for several days, especially older/weaker patients, need to be "weaned" off the ventilator. Because it has been doing both the inhalation and exhalation, the patient's lung muscles, which were used to working all the time, non-stop, can get a little soft and easily tired if asked to suddenly go back to doing everything at the flip of a switch. A doctor or RT may gradually decrease the minimum number of breaths per minute that the machine will make for the patient, and put the machine into a mode where the ventilator will wait for a patient to initiate a breath on their own, jumping in only if their rate of breathing drops below a required minimum per minute.

There's lots more settings, alarms and technology behind a ventilator, and I'm trying to keep this short and easily understood, so that's the basics of a ventilator.