They are wedge shaped and usually made of rubber. You stick it under the door, hold it in place, then pull the door to get it stuck on the doorstop. Some doors also have built in door stops that are different- more like a kickstand on a bike.

There's also the folk tradition of just cutting off an angled piece of scrap lumber to use.

There are, and they'd make fairly poor paperweights because they aren't typically very heavy - the intent is to use friction with the ground to keep the door from moving, rather than just weight. For instance:

https://www.homedepot.com/p/Everbilt-Heavy-Duty-Walnut-Brown...

There are heavy ones (cast iron doorstop will find some) and they work better in certain cases.

You can also use a doorstop to wedge a door slightly open so it won't close and auto-lock.

If the door has a deadbolt as well as a lock, you can use the deadbolt to ensure that the door doesn't auto lock by locking it with the door open.

Generally they are just a wedge-shape that gives enough friction at the bottom of the door so that the weight of the door or strength of the closing mechanism can't overcome that friction.

I suppose another folk solution is removing the pin in the arm of the device that automatically pulls the door closed. I had a high school teacher who did that.