Determining generator loads
Easy. The bog-standard way of provisioning 400A service to a residence is to fit dual 200A panels.  Because 200A gear is readily available at sane cost (e.g. $120/panel) in the consumer space, whereas true 400A gear requires you go into industrial-tier arcana, with industrial-tier pricing (i.e. nobody blinks at $3000/panel).
In your case, from the equipment and wiring used, and the breakers, it's obvious that they ran two feeders to your house.  One of the feeders comes straight off utility and feeds one 200A panel.  The other feeder comes via the transfer switch and feeds the other 200A panel.
If the installers knew what they are doing, they were careful to put loads you'd likely to want on generator on the generator-enabled panel.
400A service to houses is usually for things like 140A of heat pump emergency heat or 180A of on-demand water heater. Those few huge loads would be in the non-generator panel, since powering them is hopeless with a generator that isn't Cat yellow.
So you should be &quot;all set&quot;; just tie the generator in at the ATS and all the right circuits will have power available.