Why do some apps, like spotify, start on my phone when I use the app on my computer? How can I stop it? I don't want it to use my data on the phone but I want to be able to use my phone at the same time.
Welcome to Android Central! When you say they're starting on your phone, do you mean you actually see the app open up in the foreground? Or do you just notice the app opening in RAM if you go to Settings>Apps>Running?
Remember that an app that is open in RAM but in the background is not actively using CPU cycles or data. Android likes to keep RAM relatively full, so that apps can open or switch quickly and efficiently. The system will therefore open apps automatically (if you haven't already), and if RAM is needed for another app to be opened, then the system will figure out which app to close out of RAM.