The way that fifth copies work is a little opaque. There’s this thing called “the vault”, which you accrue progress towards opening as you obtain fifth copies of common and uncommon cards (and by opening packs I think?). Either way, they hide your progress towards it opening until you get to 100%. When you do open it, you’ll find it has wildcards in. For rares and mythic rares, there’s duplicate protection to prevent you getting fifth copies. If you ever get a complete set then you get gems instead. Hope that helps.