Making a random image generator on Swift, but being able to modify the drop rate

I want to make a gacha system, so making a random generator of images (or 3d objects), and I'm already aware about how to do this IF all the images need to be equally rare.

But just like in a gacha game, I'd like to insert drop rate, like "common, uncommon, rare, epic" ecc.

any idea on how to do this in Swift? thanks

I didn't actually triy anything cause I had no idea.