FWIW, you can buy specific skins that are on the character loadout with very few exceptions. I've never paid for premium in the game, just used the in game currency.

The confusion with Apex is that there are 3 types of currency. One is the in game free currency. That's typically used to unlock characters who cost 12,000 units. The second currency are the gold coins, which are used as premium purchasing power from the devs for things like the battlepass and specific items in the rotating store. The third currency refers to materials. Materials can be used to purchase any skin/celebration/quip etc. Materials can be bought with gold. I believe that you purchased the wrong thing.

If you really want a particular skin it's not that hard to get!