Stupid tax question

I have a really stupid question. Cause I keep finding conflicting things. Do I need to prove to the IRS my mileage? I would just manually input into stride. Cause using the app at the same time with any gig map/navigation would lag my phone so bad. Someone said it has to be proven then others say no cause IRS knows more or less how much drivers typically drive.

Secondly. If my mileage deduction is for example $300. Would I just put $300 for expenses? I was gonna go to a tax guy I know. But I can’t afford him or any other place