Using Time Machine for one Mac, one external, and two users

System: Catalina 10.15.4, external Firewire, MacBook Pro

Weeks ago I added a log-in to my Mac for my son. I'm an admin user; I set him up as a standard user. Today I went into the external's Backup.backupdb folder and saw that in Users, his folder was there -- with his permissions and with a time of a half-hour ago. Then I logged in as him, and saw that Time Machine was enabled and had the same time of a half-hour ago. Until today, his user name has not been logged in for weeks.

So, is Time Machine executing the backup from his log-in without him being logged in; or is Time Machine backing up his stuff while I am logged in as me? In a way it doesn't matter since it's working fine, but I'd just like to know. Thanks.