atf fluid level check

Here's the repair manual page specifying the transmission fluid level check procedure.

Transmission Fluid Level and Condition Check — 2019 Buick Envision Base Service Manual ~ LEMON Manuals

From the manual:

Drive the vehicle in second gear until the fluid temperature is at the specified temperature.

Driving in second gear will cause the transmission to spin faster, generating more heat and raising the fluid temperature. I don't know if your car is equipped, but the manual says you can check the temperature using the Driver Information Center.

observe the transmission fluid temperature (TFT) using the Driver Information Center or a scan tool.

A tool like the Torque app connected to an ELM327 dongle will let you read the temperature with the car running if your DIC doesn't show it (or you don't have a DIC).

As far as being level, there isn't enough difference between sitting on level ground and sitting on a lift to matter; the frame should be level in either case unless you've done something like put oversize tires on the rear or a weird situation like that.