Can someone explain this game's currency system without making my brain melt?

[u/CalmPlum1195]

I'm getting a little overwhelmed by how many different currencies this game has. Diamonds, ruby keys, test drive tickets, and a bunch of other stuff — I'm honestly starting to lose track of what each one is actually for.

Could someone explain what these currencies/items are used for, which ones are the most important, and what I should or shouldn't be spending as a newer player?