The factory did not maintain strict wire color coding. Color codes used were not the same across all models. Start with the diagrams in your owners manual but expect surprises. There is no limit to what the commissioning dealer and a series of owners may have done.

Most devices are simple 12 VDC with a positive wire (usually red) and a ground wire (usually black). Complex kits usually have detailed instructions and proper connectors. If it is something like a VHF radio or charger / inverter, refer to the original installation manual or find it online. You’ll find wiring diagrams online with many products and in Tech Articles.

No. In boats built in the 80's or earlier, wiring was usually bonded in place during construction. Newer boats often have PVC pipe "chases" bonded below the cabin liner for running behind or below the stove, for example. Except where there is an obvious chase, new wires must be run externally. Hide them bundled with existing wires up under the trim at the hull/deck joint or cover them for a seamanlike installation.