I know they can outdoor, but can they also do it on indoor maps.

I always thought they could, but was recently told i was dumb.

They absolutely can. Being on an indoor map doesn't in anyway affect a figure with Great Size from drawing LoF from any height of elevation it desires.

The only thing that changes in regards to how giants/colossi draw LoF on an inside map is that they're not able to draw LoF to *each other* over blocking terrain, which they can normally do outside

This is exactly what i thought.
My opponent said that they cant look over the terrain because
"its indoor and the map is all one level, the object goes all the way from the top to the bottom"

I tried to find something in the rule book to say otherwise, but the LOF chart only has blocking terrain and not hindering or objects.
I tried to say that if fliers have enough room to fly over people then giants have enough room to peek over the top of a garbage can. haha.

You should have pulled out one of all the indoor maps that has multiple levels...