Welcome to HF!
How is my sandbox build up?
Your sandbox (a new term which used to mean your domain) is a large simulated space that when you first install the Hgh Fidelity package, that installation creates a sandbox in your PC. So, the space you are wandering around is physically in your PC initially.
Is it gone when I switch of my PC,
It is not accessible by anyone when you either switch off your PC or disconnect it from the internet.
where is the data stored?
If by data you mean your entities (the things you see in your sandbox), initially it is stored in your PC too. It is possible to store the data in other servers you choose, and also possible to host your sandbox in another server, but that takes a little more work.
Do I live in a giant sphere, what are the boundaries?
The sandbox (also known as a domain) is a simulated cubic space 65535m per side. That is about 41 miles per side.
How do I start to build my own environment? (I can import models).
Unlike the legacy grids (like SL), there are no substantive in-world creation tools. Yes, you can rez cubes and spheres, and stretch them, but that’s about it. The expectation is you build out-world using tools like Blender or Maya then import them.
How do I as creator inspect my builds,
The default interface app (if you are from SL you would think of it as a viewer) has an entity editor. Look at the bottom of the interface window and you will see an  icon. Click it to enter edit mode, then click on an entity. You can then move the entity around, rotate it stretch it, etc. There is an editing pane that lets you inspect the properties of that entity.
how does independent and entity view works.
Much is described here: https://readme.highfidelity.com/docs/uploading-and-editing-in-high-fidelity
Those are camera viewing modes ( http://jsref.docs.highfidelity.com/docs/setposition ):
- 1st person, you see from your avatar’s eyes.
- 3rd person, your camera is set a small distance behind your avatar
- mirror, your camera is trained on your face
- entity view, your camera remains fixed (note that edit mode has the means to adjust your camera)
- independent view, camera is script controlled (read the link above for details)
I am playing around for a day now, reading the documentation and fora.Scraping all those bits and pieces together gives me a bit of understanding.
What I would like to find is a basic description how the world is build and how to move in it.
Yes, it is quitea bit different than other platforms. A good place to start is here: https://readme.highfidelity.com/docs/getting-started-1
How to build something with collissions (I keep falling down). Gravity field?I want things to be logical, not a mystery.
This is a messy area still. There is no automatic creation of a physics shape. You have to do it on your own.
Sooo much more to ask…I keep on trying and errorring
Hang in there!