there is the file /boot/firmware/config.txt. Simply add:
hdmi_force_hotplug=1
hdmi_drive=1
hdmi_mode=27
dtoverlay=vc4-kms-v3d
video=HDMI-A-1:1280X800M@60D
This programs the HDMI ports to talk to an HD-ready monitor with less framebuffer size than 1920p and less CPU time for drawing. There does not need to be a monitor connected with force_hotplug to allocate the display framebuffer and start rendering graphics to the output.