DeletedUser433 I believe that beyond a limit (maybe 3 profiles?) logging into another one causes one of the previous ones to be put at rest. It is also possible to manually put a profile at rest via the "End session" button in the power menu.
When multiple profiles are active, standard Android app power management is in force. This means that apps are allowed to run briefly from time to time, especially in response to outside events, but without special permissions an app that runs "for a long time" will be paused. Also, memory pressure will cause Android to gently stop some apps (they have the opportunity to run briefly to store what they were doing, so that when they are re-launched later they can present the user the illusion that they weren't stopped).
In general, most of the time zero apps are actually running, not even an app that is being displayed. Some games run a large fraction of the time, which is why the device can get hot and the battery can drain very quickly. Likewise, a video player may be running a large fraction of the time.