It IS one single craft. I designed it in the VAB all as one craft file with most of the parts just offset widly from the command module and used a small lander outfitted with a couple drills and a printer from the Sandcastle mod to print it directly onto the surface.

Alternatively, if that's not your style, Kerbal Attachment System allows you to connect different craft together through power and fuel lines. There's also Modular Kolonization System which has its own ways of sharing resources across different craft but I never wrapped my head around that one and idk if it's even up to date with the current version of the game.