good question Im going through this . Trying to stop them that is. Has taken control of my device. decides what texts I can recieve or send. Not allowing phone calls through. Now currently going through screen switches and other party/imbecil has opened up my hotspot for anyone to connect all for the love of meta, camera, and microphone..Hope theres an answer on how to stop the jerk doing it.
I was going through the same thing it happened for 4 months stright. I lost so many pictures videos and music because of it. I kept having to factory reset it hard reset it leave it off with the sim card out for a few hours switched to a old phone for a couple weeks and mess around with developer options and hidden menu options. Now it's working fine Galaxy A14 5G
You could try going to settings, than apps, and look for something like update manager, which is a common name used to disguise spy apps. Some things to watch for would be an app that wasn't downloaded from Google play store or Galaxy store which can be found if you click on an app and scroll down to the bottom. It can be tedious but it's possible to find it that way without resorting to doing a factory reset on the phone which is another option. Also make sure Google play protect is turned on on your phone because it's very helpful in preventing and/or alerting you to possible Spyware. A factory reset is an easier method although it's not foolproof and you'll of course have to backup the things you don't want to lose but if the person accessed your phone once they'll likely do it again so I'd recommend getting familiar with other things to look for instead. The things I mentioned are a start and I hope they help.
