Ok, maybe this is a question with an obvious answer but I am confused... up to this point I have been taught by Duo that "that/he" is expressed with वह but now I am shown उस... what is the difference?
उस is the oblique form of वह. Try to see "वह घर" as one unit which as a whole is subject to case changes.
वह घर - That house.
उस घर का/में/से/तक - Of/In/From/Up to that house.