How Do Their Amino Acids Fit into Proteins?

The invariant moieties of the amino acids incorporated into a protein, minus water of polymerization, comprise what is called its backbone, an elongated and repetitive structure in which each invariant moiety remnant incorporated forms a unit identical to every other (although of course the end-units each bear a free bonding group unused in polymerization).

And the prosthetic groups of those amino acids become protein side-groups projecting from those protein backbone units and that backbone.