I have no prior experience of advantage rules since I have never played 5e. So I have some questions.

Firstly, with critical hits/misses. If you have advantage and roll 1 on your first roll, is that still automatically a miss? In other words, do you get a second roll to try again or not? I presume not, since I quite often critically miss with advantage, which would be a 1/400 chance if I got a second roll (since I would have to roll a 1 twice). It is also a bit confusing since a critical miss still shows the to-hit calculation in the log (i.e. it adds the bonuses etc before declaring a miss).

Conversely, if your enemy has disadvantage on you, but rolls 20 for their attack, is that an automatic hit?

While I am here, I seem to recall (though it has been a while) that critical hits do extra damage, but I don't see any extra damage in my game. Is it now just an automatic hit with no bonus damage?

Do advantage and disadvantage cancel out? So if I have disadvantage for some reason, but then move to height advantage, does this remove the disadvantage? I seem to find quite often situations where I expect to have advantage (e.g. height advantage) don't have it in the log. Is this because some other disadvantage (light low light) is cancelling it out?