New backups are stored at ./incomplete until rsync exits successfully. Then the current backup ./current is replaced by ./incomplete.
The --link-dest flag of rsync always points to ./current. If the rsync command fails (e.g. because of a network failure) the backup will continue on the next run with the files already in ./incomplete.
If no backup exists at ./%Y-%m-%d_%Hh (e.g. ./2022-10-11_17h), this is the first backup of the current hour and a hard linked copy (via cp -al) of the ./current backup is created.
The same logic applies to daily and monthly backups - a copy with name %Y-%m-%d or %Y-%m is created for the first backup of every day or month respectively.
The backup retention is defined by the number of most recent hourly and daily backups to keep. This is handled by deleting all but the n latest folders matching the naming convention of hourly or daily backups respectively.