hm random but i just had barely got the halo series halo 3 and finally halo 3 odst and i was very confused on how multiplayer worked as i also just got an xbox360 a month before christmas

uh i beat halo halo 3 and halo 3odst but the multiplayer on halo 3 and halo3odst confuses me?

do you have to actually know people such as gamertags or people on your friend lists or is there not a way to join a public server as such?

i know halo3 and halo3odst came out along time ago but i barely got an xbox360 and got around to playing them uh very confused here if anyone knows or could just fill me in on how this halo3 halo3odst multiplayer works?