So my initial simplified drawing wasnt too far off. Here a less simplified one.
Now I get the logic behind the Mcl monocoque shape.
1-Like RB, they pushed the cockpit far rearwards, which brings a couple of benefits at least (that lets you increase the size of the tunnel mouths laterally, and also longitudinally, thanks to more backward low side impact structures)
2- To avoid pushing the heavy PU too far rearwards, Mclaren opted for a wide fuel tank and to not use big tub undercuts. (Which should lower the center of gravity since the fuel sits a bit lower in the tank) ...
But RB uses big tub undercuts, and unlike MCL who placed the lower SIS inside the sidepod, RB placed it much lower inside the floor (amongst many other organs of the car), to have big those sidepod undercuts
Old pic from Sam Collins from the beginning of the seaon