AC/Auto - Why does it default to having the vent open? Is that necessary?

Sure you can. Set it to whatever you want. I think the Fresh/Recirc learns what you want eventually, and the default can be set by the dealer (or yourself if you have Techstream) to your preference too.

Intuitively, you'd expect that using recirc would heat faster and also when using the cool air when it is hot outside it is better to be on recirc too.

I do believe that when you use the front defrost it will flip to Fresh air by default.

I tend to use recirc almost all the time due to old gas bangers, diesel SUVs and trucks spewing their smell and/or pollutants everywhere.

There is no heat pump in the Gen 2, that came later - not sure in which Gen.

 

Instead of a mechanical compressor, driven by the engine frontend accessory belt drive, it's just an electric compressor which operates also without the engine running. But it's not a heat pump like in newer electric cars.
The A/C Auto switches automatically to recirculation for a while, if there's a big difference to heat up or cool down. But you're free to support this by using recirculation with lower temperature differences. (Beware of the CO2 at longer trips and let some fresh air in to avoid tiredness or a little nap )

SM-G950F ?

 

Nah we keep it on recirc so wen we exit car it's like a Tommy Chong exit he he . And the AC works better too we know usually use heat

SM-A715F ?

 

Never noticed this to be a problem, but I never drive more than 4 -5 hours at a stretch on a trip.

 

Even on recirc. there is always some outside air coming in.
That is a safety feature to keep a slight positive air pressure in the cabin
to help prevent sucking in fumes from an exhaust leak.