Confused on OPTIBOOT types and usage

Excuse the newbie question. I've been all over the forum, but still confused on the Optiboot usage.

It appears the Arduino IDE boards are setting fuses for a 512 byte bootloader, but is it the same as the one shipped when buying Atmega328 chips with bootloader?

I see folders for the IDE, that have the Optiboot listed, but I don't see that on my list of boards (they use a bootloader, but not referencing the Optiboot files).

I just want to have the IDE using the latest, refined version of .the bootloader, when I select various boards.

I just can't seem to sort out what it's using, and why the 'optiboot' folders aren't showing up on my board listing (should they?).