No power to anything

Start by opening up the big power/ relay box under the hood. With a light or meter, ground to engine block, probe the two large studs. Both should be hot. Probe the big stud on the alternator, and that should be hot.

Measuring large power cables for resistance may be throwing you, as corrosion internally might make enough connection to read on an ohmeter.

You need to check stuff like that under power

Also wiggle some stuff with the key on and door open, and listen, watch for dome chime/ buzzer etc

Inspect those huge fuses I don't know what all they do without looking it up

I would think this is a "main" problem as those are so well fused, it would take a "key" circuit to dump everything.