Canon EOS R3 Advanced User's Manual page 336

Autoexposure Recording
Exposure is controlled automatically to suit the brightness.
1.
Set the recording mode to [
2.
Focus on the subject.
Press the <
Before recording a movie, focus with AF (
By default, [ : Movie Servo AF] is set to [Enable] so that the
camera always keeps focusing (
When you press the shutter button halfway, the camera focuses using
your specified AF area.
].
> button, then turn the <
).
336
> dial to select [
) or manual focus (
].
).