This is for a house but explains what a main is.
So, what is a main breaker anyway? You'd think that it is something special having a name like that. Actually, the main breaker is the breaker that the feeder wire connects to. This breaker is a two-pole breaker that is connected to 240 volts to power your home. The main breaker acts as the disconnecting means to the entire power load of your breaker box. With it off, there is no power being fed to the buss bar that feeds the branch circuit breakers.
I didn't understand your question until the others answered. Sorry, I made an assumption that everyone knew what a main did. I forget not everyone pokes around in their panels.

As for the red 12v yes more amps. If it you look at them you can tell its also heavier gage wire. Mine are not used I only need/have 2-10's one for water pump and one for 12 volt outlet. All my lights are 12v they're on 3's all bulbs are LED's. I have only 1 thing wired to each fuse and breaker. Since I used dual breakers I have an unused slot. So I did put a dual 15 in the slot shut off as a spare. Or if I add more wiring later. I/
You can wire more than one thing to each breaker or fuse. As long as they do not exceed the amps of the fuse/breaker. I could wire all my lights to one fuse- 7.5 or 10. Or I could wire my 2 receptacles (outlets)to one breaker. Again as long as what I plug in to them doesn't exceed the breaker. I did one on one so if I need to add more outlets I can just add a wire and outlet to the load terminals of my GFI outlets and they'll be GFI protected.