First up are the 4 infinity cool ducks. 3 of these just kinda spawn in naturally, while the other requires the player to do stuff. I'll show the 3 first.
Computer Mouse Duck:

Computer Duck:

Gamer Duck:

You can get these ducks just by waiting, over time (~2-3 in-game days) and they will be brought in by some kind of mini spaceship or UFO.
Image of the gamer duck being dropped in:

(Don't forget you can click on the screenshot to make it larger)
QR code duck:

To get the QR code duck, there might be some spoilers, but if you try to solve it yourself, it would be very difficult. To get to this duck you need to open the vault.
An image of the vault:

Where it is compared to the pool:

If you want a more detailed guide on how to do that, I suggest this one: https://test-steamproxy.haloskins.io/sharedfiles/filedetails/?id=2952386429&searchtext=door
If you don't feel like doing that, I will give you a very basic and bare bones answer.
The code to the door is unique for every person, to get the answer, name the psychedelic duck "LSDina" and the black and white cartoon duck "Hedy" Naming them that will turn their names into a 2 digit number. The psychedelic duck's number comes first. Enter the code into the keypad and once you are in, grab the soda if you want a secret collectible, then go to the monitor/computor and press the duck button. This will spawn the QR code duck in the pool.
Congrats, you got every Infinity Cool Special Duck!

There are a total of 3 ducks in the Quack in the Ice DLC.
First up is the Lumberjack Duck:

He spawns in after the second time the Zamboni (the car in the icy area) exits the garage, this duck is the easiest to get in Quack in the Ice.
Next is the Ski duck:

They are encased in a cube of ice on a tree. Here is a picture of it:

To get them to fall off, you will need to wait for a woodpecker to come a few times. The woodpecker:

Once they fall off, you will get the achievement for the Ski Duck falling down, and they will be added to your collection. If you want them out of the ice, you need to make the pool a hot tub, by having the weird circle fire place by the area the lift drops ducks in, lit on fire by a duck. (this will give a achievement for making it a hot tub)
Finally, it's time for the St. Bernard Duck:

(for some reason he likes to be upside down on the lift) To get this good boy, you will need to have a (non-propeller hat) duck fall off the mountain (this happens most often by more than 3 ducks getting into the lift ramp and a duck does not make it before the door closes, falling off the mountain.) The dog and the fallen duck will appear on the next lift. I've seen the dog exiting the pool after a little bit to rescue more ducks, but haven't experienced this myself.
That's all the Quack in the Ice Ducks!

Like Quack in the ice, there are 3 ducks here.
First is the UFO Duck:

To get this unidentified duck, you will need to first click on this big robot thing:

Then you need to press the big red button to launch the duck in there (if there is):

The duck will be launched towards this big sphere:

On the sphere there is circle with a X on it. In the middle of this X is the UFO duck:

You will need a duck to be launched and hit within that X circle to free that duck. Once you do this, the duck that just got launched will be stuck to the X, waiting for the next launched duck to free it.
Next is the TV Duck:

To get them you need to get a duck to go through the tunnel while the cow tentacle thing is moved. It moves every once in a while. The cow tentacle thing blocking the path:

Once a duck is through, it will be inside the weird sphere thing:

Click the computer/console once you can and you will be greeted with this:

Click the grey button in the top right to release the TV Duck. If you want to learn more about the TV duck, then I suggest reading this guide:
https://test-steamproxy.haloskins.io/sharedfiles/filedetails/?id=2986626145&searchtext=tv
Finally we have the Astronaut Duck:

It has something to do with ducks going through the portal in the sphere and it's caused by a duck already being in the launch robot. The portal:

There is a secret involving the TV duck, the Astronaut Duck, And the computer inside the launch robot. If you want to learn how to do that and get the secret, then I suggest this guide:
https://test-steamproxy.haloskins.io/sharedfiles/filedetails/?id=2986509134&searchtext=code
Congrats! You got every Hippospace Download Duck!

This uses the So Many Ducks DLC. Here they are:

To get them, first you will need the robber, piggy bank, and arcade duck in you pool.
Robber Duck:

Piggy Bank Duck:

Arcade Duck:

First you will need the robber duck to steal a coin from the piggy bank duck:

Then you will need him to give the coin to the arcade duck:

All you need to do now is click on the arcade duck to activate it's minigame:


.........It plays like that no internet dino game, and to get the Boxed Duck, you will need to obtain a soda, which spawns somewhere random in the level. Once you get it, the Boxed Duck will spawn in every pool you have.
BONUS: If you go all the way to the end in the mini-game, you will get a randomly generated 4 digit number. If you name the lockbox duck (the one that looks kinda like a aged blue robot) it will open, giving you an secret paper.

First up: Window Cleaner Duck

He will come naturally, after a short period of time, I assume they ride up on one of those window cleaning lifts, but i didn't see it myself. (Note: i found out the duck rides up on a string, coming from the weird orange rectangular box at the top)
Next: Elevator Duck

You can get them when the first duck rides the elevator, since they will be waiting in there. You will only officially obtain this duck once the elevator ride is over.
Next: Pigeon and Hawaii Shirt Duck

These two will be in the little kiddie pool at the top, once you come out of the elevator.
Next: Business Meeting Ducks

After a while these ducks will have their little business platform once they finish their meeting.

Finally: Brick Duck

Little bits of the brick/concrete/whatever from above will break off.

(look high to see where it is chipping off)

(some of the debris. (im like 99% sure these despawn))
Anyways, after enough stuff have chipped off, the duck will fall off too.
That's all of the ducks. That was pretty easy, just wait like ten minutes, and you will probably get all of them.

This is what your collection should look like with all the special ducks:

Feel free to like and favorite if this was useful!