How do new players get the garrison?

Hi!

New player around and I have a question whose answer I can't find. I'm new (actually, still at the free trial up to level 20) and I've been reading some threads in reddit and other forums.

I have read about something called Garrisons, that are the closest aproximation to housing in World of Warcraft. I'm actually leveling in Dragonflight through Chromie because it seems that in dragonflight you get unlocked some kind of flight skill.

How do I unlock both things? Am I supposed to only get one of those because I level on one on another expansion?

By the way, sorry if my english is not up to anyone's standard. It's not my first language, hahaha.