One Hot Apple

My iPhone 12 Pro Max suddenly started having severe battery drain and starts getting hot; not warm; during normal use. I did a FACTORY reset; not settings reset, which I thought wiped the phone; but now the issue seems worse. Phone is in near-mint condition and in a mil-spec case. Never had over-heating issue before so this is out of the blue. Also heats up while plugged in. Don’t have money to throw at it so I need a FREE solution or I’m making apple pie. :)