Multiple for loops executing at the same time? Swift - swift

Question

Basically I am creating a game that spawns a ball after a certain amount of time has passed. However, as time goes on and the player score increases, balls that are more difficult to kill will spawn.
Prior to this, all the spawn methods I have set in place function as I would like them to, but they were contained in a switch statement that calculated the probability a ball would spawn.
//pseudo:
let spawnProbability = arc4random_uniform(100)
switch spawnProbability {
case 0…60: //60% spawn chance
//spawn small ball
case 61…90: //~30% spawn chance
//spawn medium ball
case 91…100: //~9% chance
//spawn large ball
default:
break
}
This worked as a plan B, but I'd like to implement what I originally had designed which was the method described above.
My thought process to accomplish what I originally had in mind is:
for loop that spawns small balls until the score counter reached a certain amount
after the score counter reached, say, 100pts, another for loop would be executed in addition to the first loop. So, medium and small balls spawn after a delay.
after the score counter reached another mark, large balls would begin to spawn on top of medium balls and small balls.
Here's what I mean: (pseudocode again)
for(var i = 0; i < 10; i++) {
//delay
//spawn small ball
}
if score >= 100 {
for (var x = 0; x < 10; x++) {
//delay
//spawn medium ball
}
}
// . . . and so on
I've put alot of thought into how I could accomplish what I want but I can't come up with a better approach to this problem. I understand why this isn't working and why multiple for loops can't be executed with delay in the same class at the same time (or maybe they can and I'm just going about it in the wrong way) but I'm stuck in the sense that I don't know a better way to implement the spawn technique that I want.. Any help is greatly appreciated!!