see under cli: /etc/pve/storrage.cnf or with the gui under Storrage.
Then you can find 3-4 files with the same name. The timestamp show you the last one.
Those are PBS-backups. The way how PBS works, there is no "regular backup file" which you could simply copy over.
Either you need to connect the other PVE also to that PBS or you copy the whole PBS-datastore to the location of the other PVE and add it to another PBS instance there or you sync (parts of) the PBS-datastore to another PBS instance with the integrated remote sync function: [1].
To get a "regular backup file" which you can easily copy over, you need to do a non-PBS-backup.