@kristinaS21: Please can you try pressing the Volume Down and Power Keys together for around 10 seconds until the phone restarts. Don't worry, this won't affect your personal data or settings. Following the restart, head to Settings > Battery and Device Care > Optimise Now, then select Memory > Clean Now. For any specific apps you're encountering the issue with, head to Settings > Apps > Select the relevant app > Force Stop > Storage > Clear Cache.
@TMIAH: If you are experiencing frequent restarts, then I recommend enabling Safe Mode, and monitoring the performance for a few hours, or longer, depending on the frequency of the issue. To enter Safe Mode, press and hold the Power button until the 'Power off' prompt appears on the screen then release > Touch and hold Power off until the 'Safe mode' prompt appears then release > To confirm, tap Safe mode.
If you notice an improvement, restart your S21 as you would normally, and delete any recently installed third party apps. I won't be possible to tell which app the problem is stemming from, therefore I recommend doing this via a process of elimination.
