Starting off, let’s look at the Marvel Comics version of the Quantum Realm, which is known as the Microverse.
The Microverse can only be accessed if an individual shrinks themselves to a small enough size, usually through the use of Pym particles.
Once there, the Sword shrunk Wayfinder and the Ithacons to subatomic size so they could build a new home in the Microverse.
In the modern era, various superheroes have traveled to the Microverse, and when they were part of Marvel Comics, this is where the Micronauts superhero team operated.