So I am thinking of building a first MTB, a first bike [EDIT: my first bike build; I have owned and ridden a number of bikes and MTBs], and i am really ignorant (just ordered a book from amazon, Zinn and the art of MTB repair, something like that), have what is probably a stupid simple question to answer: so as I look at MTB frames on ebay, I am wondering do I have to look for some special size of the holes in the frame where the bearing for the crankshaft/pedals will assemble, and also where the steering tube will go in? Or is this kind of a "universal fit" thing, where all frames pretty much have the same size holes for attached components?

Same question for a MTB fork. Will pretty much all forks fit in pretty much all frames? or does the O.D. (outside diameter) of a fork need to be matched to the I.D. (inside diameter) of that hole in the frame?

Thank you for your patience in a noob's dumb question!