Looking for information/schematics on the load bearing points on the roof.

I am considering adding full length rails to the roof using strut channel, to which I can mount just about anything. But I'm worried about how much weight (and where) the roof can actually support?

It has the factory sunroof with small rear roof rack like packers15 shown here: My 92 4runner

I see there are also ways of mounting racks in the front, like this: https://www.toyota-4runner.org/1490106-post13.html But this looks more like external clamps, not bolted into the roof.

I am thinking if I pull off the factory half rack in the back, I can probably use those anchor points, but need to add at least 1 anchor point in the front near the top of the A-pillar. I can't tell if there is any frame rail under the sheet metal of the roof, and what would be a good location for this?

I've been searching around and can't find schematics showing what the structure is like "under the covers." I do see people talking about nut inserts into the sheet metal, but not sure how strong that actually is.