I have a mom question about car seats/strollers

I'm 14 weeks pregnant with my first, and doing my best to research all the things we need for our little dude! I'm a little bit confused about car seat safety and strollers. I've heard that you are not supposed to let your baby sleep in the car seat, but what if they fall asleep in the car? I'm also confused about car seats that fit into a stroller, because if they aren't supposed to sleep in it or be left in the car seat for an extended period, how is that different than being in the same car seat on wheels? Do most people have car seats that attach to their strollers? I feel a little silly asking but I just want to make sure I'm doing the right thing!

Edited to add a word, I forgot the "not" before supposed to! Oops!