In Ant-Man and the Wasp, how did
Janet Van Dyne live in the Quantum Realm? How did she get the hood, the spear... the makeup?
Do they explain it in the movie?
Nothing is explained.
It appears that some propery of the Quantum Realm manifests as some sort of healing energy. This is presumably what sustains Janet.
this is what Janet uses to help Ava in the conclusion
We know there's some sort of mental connection involving the Quantum Realm. Perhaps this also allows Janet to manipulate her environment, making it livable and creating clothes etc.
In general, there's no science or logic to what happens in Ant Man. You just have to accept the premise that there's this cool size-changing stuff that works however the writers need it to, and go with it. It's superhero Action/Comedy/Drama, not SciFi.
I think the answer is hidden in the word "EVOLUTION". Van Dyne uses this word when she says, "this place changes you". I think she means that since time and space work very differently in the Quantum Realm, then the way we humans evolve would work differently too. Somehow her mind and her body evolved to survive the new ecosystem she had to survive in. This is one thing we humans are good at - adapting & surviving and these two things bring evolution.
This fascinates me because of a free VSauce YouTube Red MindField episode I watched a few days ago. This video talks about a theory that seems appropriate.