Easy to code, these don’t need to use XPC although it’s used by the DAS-CTS scheduling and dispatch system. They aren’t run at constant time intervals, but when appropriate according to other loads.
zombie
If you leave some apps without an open window, macOS appears to quit them, but instead puts them into App Nap, only to quit them when it wants to. They’re undead.
The odd behaviour of some apps, which quit on their own, but hang around as zombies, and the long battle trying to make sense of logs.
The Eclectic Light Company 