Pixhawk 6X is shipped with PX4 FMUv6X Firmware
Ardupilot Firmware can be flash via Mission Planner or QGroundControl. It can also be downloaded here: https://firmware.ardupilot.org/
PX4 Firmware Target: FMUv6x
Ardupilot Firmware Target: Pixhawk 6X

Rev 8 (ICM-45686) Version
PX4

Support in PX4 1.14.3 release and later.
Ardupilot

Supported in 4.5.0 stable release and later.
Rev 3/4 Version
PX4

Pixhawk 6X is supported on PX4 1.13.1 release and later.
Ardupilot

Supported in Ardupilot 4.2.3 stable release and later.

REV3 & 4 not supported in Ardupilot 4.5.3, please use 4.5.2 or 4.5.4.