OneDrive offers several options. I think it defaults to wanting save space on the local computer. So it only keeps stuff on the hard drive that you are actually accessing.

I have one computer set to always have off line copies of everything on OneDrive. That's my main personal laptop. On rare occasions that I remember, I can do a full backup to a local backup drive.

Another computer which is used for work, I have it set to only have access to certain folders, and just basically only have the files on the cloud.

Then there is also the watched folder concept where it monitors and backs up some local document and picture folders. For those people who aren't used to monitoring where they save files to, this is probably the best option. I haven't really poked at it thoroughly.

I guess I learned Dropbox after OneDrive. I can't get along with Dropbox as well. Probably just uses slightly different terms. Or, it was because I only used it when someone shared something with me.