I grappled with this issue 2 years ago when I moved to Linux due to work. (Was using onedrive personally).
OneDrive doesn't have linux sync, open source solutions didn't exist either.
Dropbox dropped support at some point although still functional for now.
https://www.dropboxforum.com/t5/Dropbox-files-folders/Future-Support-for-Linux/td-p/292412
I don't know about Gdrive but I don't think that was easy either
I ended up going with Open Source solution "NextCloud", basically dropbox clone. Nextcloud has client for Linux, Mac, Windows.
You can either use one of companies that provide this server, or host the software yourself on server you rent. I pay $80.12 for 3TB storage per year.
Typically NextCloud option is more expensive compared with massively good value storages like OneDrive but you probably have better privacy also.
If you're after purely backup there'll be more options then "sync" type of services.