What can be causing my phone to turn off randomly?

I have a galaxy note 9 and lately it is been turning off randomly. Some times it won't even turn on all the way. I don't think it's the battery because when I open an app and start using it it doesn't turn off on me, it only turns off on me when the phone is not in use. I have iHeart radio app playing nonstop on silent so it won't turn off on me lol

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