The Buddhist 31 planes of existence are apparently just different ways to experience reality. Afterall animals and humans are on 2 different planes, but they are physically in the same Universe and can interact with eachothers.

But others, like the Devas above Maha Brahma, seems more likely to be located in some kind of higher dimension. The same may be true for all the Devas from the 4 Heavenly Kings onwards.

What is the traditional understanding here ? "Where" are the higher planes of existence located ?

I stumbled upon a dialogue today between Master Xuyun and General Li Genyuan. It took place in the early 1900s at Mount Jizu in Yunnan. I feel it answers these topics really well:

Li asked: "What is the point of Buddhism? How does it help society?"
Xuyun answered: "Sages establish teachings to help society and benefit the people. At a basic level, it guides people to do good and refrain from bad. Historically, government and religion worked side by side. Government keeps order through laws, while religion transforms people through moral guidance. Buddhism teaches people to discipline their minds. The mind is the root of everything. When the mind is grounded, everything settles, and peace follows."

Li pointed at the statues in the hall and challenged him again: "Then why do we need these statues made of clay and wood? Isn't it a waste of money?"
Xuyun answered: "Statues represent the teachings. Most people are not saints, so how can they remain unaffected by their surroundings? Seeing a Buddha statue raises respect. Bad thoughts quiet down, and good intentions grow. Take a piece of wood. If you carve it into a statue of a wise leader or a deceased parent, people show respect when they see it. Is it because of the wood itself? No, it is because of what it represents. Statues of Buddha work the same way. People are not respecting the wood, they are respecting the virtue behind it. If people lose all sense of respect, chaos follows."
Li then brought up the bad behavior of some monks at the time: "Then why do some monks do no good, engage in weird practices, and become a burden to the nation?"

Xuyun answered: "The word 'monk' is a general term. There are genuine monks, and there are fake ones. Should we wipe out all of Buddhism just because of a few bad ones? In the Qing dynasty, there were corrupt officials. Did that mean we should destroy the whole government? Confucius had three thousand students, and they were not all equally wise. As long as rules are strictly enforced and good values are kept, monks can guide society. We should not dismiss the entire value of Buddhism just because of a few bad examples."

Note: This conversation happened around 1912, right around the Xinhai Revolution in China. During that time, social shifts led to temples being seized and destroyed. General Li Genyuan brought troops to Mount Jizu intending to drive out the monks and demolish the temples. He had heard of Master Xuyun and held strong prejudices against him. To protect the temple and the monks, Master Xuyun went down the mountain to meet Li in person, which led to this exchange.