A Microsoft file hosting and synchronization service.
This has happened to me as well.
Started 4/1/2025 (for me). All folders that were shared to me, and synced to my desktops, have disappeared from my desktops and replaced with URL links to OneDrive instead. Also, some new folders were created with the original name plus the PC name added to it (as others have experienced). On one desktop I noticed it took some Windows updates early in the morning (around 12 AM per Reliability Monitor), but the OneDrive folder shenanigans didn't start until that evening around 7:20 PM (based on timestamps shown for the folders which were moved to my Recycle Bin). The shared folders no longer appear in the OneDrive app as selectable for syncing - they are not showing at all in the list of folders though they are there in the web OneDrive.
Hopefully Microsoft gets this sorted out soon as it's made using OneDrive for sharing between family member accounts difficult.