A Microsoft file hosting and synchronization service.
OneDrive Backup works by syncing content between OneDrive and your local folders. If you want to have some files on your local PC but not on OneDrive, move that file to folder that is not synced to OneDrive (outside Desktop, Pictures, Documents, ...).
Also, very important is to understand OneDrive Files On-demand. To save space on your local PC, some files are available only on OneDrive, although you can "see them" on your PC. When you click on such a file, it will download it first from OneDrive. Please check this article for full explanation: https://support.microsoft.com/en-us/office/save-disk-space-with-onedrive-files-on-demand-for-windows-0e6860d3-d9f3-4971-b321-7092438fb38e
You have an option to always keep all files synced (Backup) to OneDrive on your local PC.
Bottom line, OneDrive backup is great, and it can be used as a backup if your PC crashes, if something happens to your file (there is a recycle bin on OneDrive, and also you have version history), etc. However, it's not "traditional" backup where you backup files from time to time. OneDrive is doing that all the time by syncing.
Hope this helps.