Rust underneath ... unsure if its structural?
 

I wanted to rebuilt Chevy Blazer 1988.. there is rust in metal below and its so bad when I press my finger against the large metal thingy my finger goes through in inside the under carriage somewhat under below of the rear seat it as shown in the picture with arrow.. The picture was taken from the right rear wheel side

I wanted to know if this is structural or non structural and are things easily repairable with any replacement components?

Would anyone guide me as wanted to do it myself as project