No, Greg ... you're making it more complicated than it is!
The "All in one" typically has a meter base whose lower lugs extend (just barely) into the 'panel' side of the cabinet. Here they immediately enter (back feed) the 'main' breaker .... which powers the busses through the usual breaker contacts.
Here's a pic of the Sq D model I mentioned earlier. OK, it actually has wires, not lugs feeding the main breaker ... but you should get the idea.
In use, the top half has a sealed cover, while the lower half is acessable as any panel would be accessible.