When you are in "Park", tap on the HVAC icon would show you the options for:
1 ) Keep Climate On
2) Dog Mode
3) Camp Mode
If you don't choose any of the above and the screen times itself out, the occupant can touch the screen to reactivate the HVAC again.
If the occupant allows the screen to go dark for a while, touching the screen won't work anymore.
In that case, opening a door might wake up the screen or specifically, must be the driver's door after a while.
If you turn on the HVAC by the app, it will time out after a while too. You then need to turn it back on again manually.