My phone randomly turn off and others problems

My phone started turn off randomly when the battery is below 15% since last year and I don't know why.

My phone started to detecting the screen by itself when the battery is below 20% and it mainly detecting the button at the lower- left corn the most hilarious thing is that when i try to go back to the app the phone keep resisting me that it don't let me go back.

We can help me pls