Phone died for a month and turn on again

Like the title said my phone Samsung s21 ultra dies last month and I don't know the cause of why. It was fine and then I put my phone down on my table for 5 minutes went to check for time and it just won't turn on. I send to a repair shop and they said they couldn't fix it. A month later now I randomly put it on charge and it worked for a day and overnight it died again for another 4 days, I replace the battery but it won't work for another 2 days I put it on charge and now it worked again. Does anyone know why that is. And I found something funny that when I managed to make it work the second time I had a YouTube video playing overnight and it was fine, as soon as I stop playing YouTube died again