Help me understand the fuel system!

Regulator........depends. If it's a return type you want a separate line back to the tank. Whatever you do, don't be tempted to install the pump up front. "pushing" helps eliminate boiling fuel / vapor lock in the line "coming up" front

Relay / switch. CONSIDER. Just what might happen if you wreck it? You can use something like a Ferd Ranger crash switch in series with the relay. You can ALSO hide a switch somewhere in the relay coil circuit for anti--theft. ALWAYS wire the thing to switched ignition so it "kills" with the key. Notice what the pump draws for power, bigger wire is always better than smaller LOL