How do I get random item based on percentage?

Here's the start of an algorithm but you'll have to test it and make adjustments. We'll assume that you're using a good random # generator that truly generates arbitrary values.

Let's assume that we're building for the silver pack and that you said 1 in 50 are silver and 1 in 1000 are gold. Normalize to 1000 so 20 in 1000 are silver, 1 in 1000 are gold and the remaining are bronze. Generate a random number between 1 and 1000. You can partition your cards up as much as you want but somehow you'll need to decide where the 20 silver and 1 gold card sit in the 1-1000 range. Suppose, for example, that you specify that the gold card is at 1000 and the silver are 1-20. If the random # is in either range then you use that value. However I think you'll find that 1 in 1000 is extremely rare even on the best days.

There are quite a few variants you can use to adjust the behavior to your liking such as pre-selecting the cards and randomizing from there or using some custom rules such as a gold pack always having at least 1 gold card.

simple brand picker:

    static Random brandRand = new Random();  
	public static string GetBrand()  
	{  
		var value = brandRand.NextDouble();  
		if (value <= 1.0/1000.0)  
			return "Gold";  
		else if (value <= 1.0/50.0)  
			return "Silver";  
		return "Bronze";  
	}  

So let's say I want to grab 4 cards at random from bronze, all cards will of course be bronze when I buy the bronze pack. If I buy a silver pack, any or all 4 can be silver, but still have bronze cards, based on percentage. If I buy a gold pack, any of the 4 will be gold, some silver and mostly bronze.

My code as I have as follows. It will need some modification, for sure.

var random = new Random();  
var items = (await _cardService.GetAllItemsAsync()).Where(c => c.Level == pack.Level);  
  
var userPack = new UserPack  
{  
   Item1Id = items.OrderBy(c => random.Next()).FirstOrDefault().Id,  
   Item2Id = items.OrderBy(c => random.Next()).FirstOrDefault().Id,  
   Item3Id = items.OrderBy(c => random.Next()).FirstOrDefault().Id,  
   Item4Id = items.OrderBy(c => random.Next()).FirstOrDefault().Id  
}  

pack.Level would be equal to Bronze, Silver or Gold.

I hope this clears up some of what I'm trying to accomplish.