There is a concept of apps running "in the background" in cell phones. When we say the app needs to be running, we don't mean it needs to be at the front end in main focus, we just mean it must not have been stopped. You can force stop apps and that would be a problem, you would have to re-launch it manually. Similarly, the operating system can suspend applications (prevent them from running in the background) if it thinks they are consuming power for no good reason. There are options to control that in the operating system, it varies per OS type, and varies per version of the operating system too.
As an example, every time I install an update to the Tesla app on my Android phone, the "phone as key" feature stops working. I need to manually launch the Tesla app ONCE, or reboot the phone. That is simply because the Tesla app must be stopped in order to perform the update, and it doesn't get auto-started after the update. Once you understand, it's pretty logical.