I know its an imponderable or unconjecturable to us putthujjanas, but do any of the 31 realms have corporeal existence or are they states of mind (mind-made) or a mixture of both? For example, does tusita heaven physically exist, or is it purely a state of mind? If tusita does exist physically then how can one get there? Is the earthly plane and the laws of physics also mind-made?
When you read the Pali Canon, they seem like places that exist that beings live in. At the end of a teaching, the Buddha would explain any similes or metaphors he used. I've never read him explain the 31 planes as a metaphor, which means he taught it as a literal existence. There are also examples of monks with past live memories talking about their previous existences or visiting other realms in this life. Maha Mogallana was known in particular for his psychic powers.
To get to Tusita, you need to create the type of karma that would lead to being reborn there. In MN 120, the Buddha explains how to influence your rebirth by aspirations. Alternatively, you develop the Divine Eye, a psychic power. To do that, practice the Noble Eightfold Path and learn the 4th jhana. That is not an easy task.
I don't think any sutta states why the laws of physics exist.
During the Mongol Yuan dynasty, where the Sakya-Mongol alliance reigned, I was a monk at a temple in Tibet. It was in the year 1247. I had a big nose and smelly breath, and I walked like an Egyptian, but I could throw dhamma bullets from my mouth like a shaolin warrior throws needles, and they liked me for that.
My general behaviour was unkempt and controversial at best, and I would often hear my fellow dhamma farers complaining about me, but they'd always finish their lamenting with, "sure, he's a weird guy, but he speaks dhamma well - let him be". The Lamas of that particular era held huge political and military responsibilities alongside their religious endeavours, which I did not envy in the slightest.
I died a peculiar death in 1261. It was precisely 49 days later that a crack team of siddhi enthusiasts tried to track me down with their divine eyes. One of those siddhi masters got too close, so I poked him in the eye with a divine poke, a special siddhi move that can be transferred into the physical world. Their confidence had been bruised as they found they were one man down for a few weeks. Regardless, they never found me, and I made damn sure!
3 weeks before I died, I deliberately vandalised a sand mandala two days before it was due to be finished as a way to teach them that impermanence doesn't follow schedules. Well, they didn't appreciate that! The sand used for the mandala had been collected from various rocks covering a 25 mile radius, painstakingly ground into sand and partially made into a rudimentary mandala, awaiting its final detailing. As you can imagine, there were some tears and a few wet robes. Mandala creation in Tibetan Buddhism was in its very infancy at that time, and they 'apprehended' the idea from the Vedas.
As you can probably incur from the style of delivery, even in my other lives, I wasn't a very conventional sort of person, and often went against the grain of things. I guess some habits never die! *wink!
My point is that all of this is a state of the mind, just like the realms, themselves. Perhaps they happened or are happening now, but regardless, they remain figments of the imagination, no matter how creative they my seem.
I do have more interesting past-life stories to share, though - even some other-realm stories! It's fun, isn't it? ;-)
If you like Buddhist cosmology, try watching Ajahn Sona's excellent 10-part series on the subject, here. It had answered a lot of questions for some of the things I was seeing in my own practice. But be warned! This region of Buddhism can captivate the mind, thus causing one to drift off from the primary objective! This was illustrated in the story of Mae Chee Kaew, who often visited these planes and spoke to the inhabitants. Her teacher, Ajahn Boowa, immediately told her to stop doing this in order to guide her towards enlightenment, and at one point he shouted at her, and for good reason! For most, it's best to steer clear!