I don't think I ever asked this but how does HFA work?

There is an algorithm that tracks crowd noise based on attendance and team momentum. If the crowd noise is high, there is a slight advantage to the home team in their attributes, and there is a slight increase in false start penalties for the visiting team.

Shouldn't familiarity with the field, altitude, wind patterns, sun, temperature come into play as well?

Wind, precipitation, and temperature do come in to play. Wind affects the flight path of the ball, precipitation effects the ability for players to hold on to the ball and to make hard cuts, and temperature affects fatigue. I haven't done anything with altitude yet, and hadn't considered familiarity with the field. That's a cool idea though.

I meant all of those things in context to familiarity. Look at how opposing kickers do worse in Heinz Field as an example.

Or how hot weather teams handle playoff games in Buffalo or Green Bay.

I've gone back and forth about that, honestly - I don't really want any of the teams to have a built-in advantage out of the box, i.e. Denver has an advantage because they're at altitude and Miami can't play anywhere outside of Florida. I know that's the way it is in real life, but I don't want to discourage players from choosing teams that will always have a disadvantage when they don't play at home; so I've tried to keep it where each team has a common ground zero.

It goes both ways...some teams just can't handle Miami sun and humidity in December.

You could also include scheduling fatigue such as in the case of teams playing on Sunday in SF and Thursday in New England. Or even 6 day weeks for WC teams.