I see all these various Bitcoin collectibles, either pre-loaded with private keys, or you load them yourself.

So for all the experts who know this stuff better than I do, what private key format is most commonly used for Bitcoin collectibles?
The mini private key format? WIF? The raw private key? Is there a standard that most collectibles use, or does it vary from item to item? Have seed phrases or brainwallet formats ever been used? Did the old Casascius coins set the standard?

Very curious about all this, as I never followed the collectible Bitcoin market until recently.