what's a bike made of?

[Maxine]

I'm starting to think that the fit and the saddle and the tires and things like carbon seatposts and forks matter as much as the frame materials.

You are starting to think correctly. Frame material, in and of itself, is well down on the list of things that make a bike more (or less) comfy to ride.

Use at least size 25 tires, and don't pump them up any higher than 95-100 psi . . . presto, lots of extra comfort for dirt cheap!