The conventional brakes engage when the car reaches deceleration rate of approximately 0.25g (from VW). Like you, I also am assuming the bright green bar represents regen charging. If you step on brakes hard the conventional brakes engage and the bright green bar goes back to zero / midpoint.
People that stop move aggressively might want to use B mode versus coasting in D mode. As pointed out on other posts, regeneration is not "free" since you have converter and motor losses into the battery and then back out from battery, which can be 20%.
The friction brakes also engage when you reach 0 mph regardless of how far the bright green regen bar has gone. You faintly feel or hear the brakes at that point. Hard to describe, but all ID4 drivers have been there I suppose.
If you read the braking system description in the NHTSA docs, there is a direct pushrod connection from the brake pedal to the master cylinder, but the system then boosts input force up to 8x.
The doc doesn't spell out clearly how regen is mixed in, but despite what VW says in the marketing material, there's no escaping that with this system the hydraulic brakes are engaged immediately when pressure is applied to the pedal. I'm guessing the amount of boost varies based on how much regeneration is being applied.