Couldn't find a sticky so, how does the Halo Reach Matchmaking system work?

Magically it matches you with other players. Basically, you look for a match, the server says "Jesus take the wheel" and much like he turned water to wine, he turns a lobby into a game.

I mean how does the ranking system work

lol

same way as in call of duty

Play 3 games. Get rated. Wake up next day have your rating reset.
Do this 11 days in a row you get a ranking.

Im pretty sure thats about it (I could be off with the numbers etc)

4 games and 4 days

It doesn't.

What ranking system?

He probably means trueskill.

The matching system in Reach works roughly the same way as it did in H3. I heard there were a few small changes, but the basic premise of the system is the same. You go "up" more for winning against opponents with a high trueskill than you do for winning against opponents with a low trueskill. You go "down" more for losing to worse players, etc. It's invisible, but it's still present.