What is the output file type (file extension) of the backup? in time machine

On macOS Monterey, or any Time Machine drive formatted with APFS, each backup is a date/time stamped folder containing the filesystem hierarchy of the current backup. The Backups.backupdb tree is gone.

"Backups.backupdb"

".sparsebundle"

Not sure, if this has changed in the newer versions of macOS.