fire/garage doors
 

I know that I should scour the codes here, but I think this would be the same in most places.

I have a detached garage. I know (I think) that when your garage is attached, you need a fire-rated door between the house and garage. Since mine is detached, I assume that I can replace the door with whatever I wish. Correct?

yup again
 

The metal doors at the box stores are 150$$ and they are insulated and usually rated as a 20 minute firedoor.
To answer your question, yes when attached to a house where it enters the living space.

Unless it's in the exterior wall within 3' of the property line, then needs 1 hour construction of the wall. IRC 302.1 That would be really friendly!
Be safe, Gary