(could be starter motor, solenoid, failed battery, etc.

My Jeep ZJ had an issue starting up fine cold but not warm. Ended up being a corroded negative terminal on the battery. You couldn't see it because the insulation was tight, but right around the battery stud connector there was the hint of copper corrosion. When I pulled it and tapped it, the darn thing almost fell apart.

I replaced about 6" of the negative wiring harness and it was good as gold ever since.

Intermittent starts are generally electrical in nature. (failing motors, solenoids and grounds). Mechanical issues are more predictable: loud starts, hard starts, etc.