overlap2d | Overlap2D Game development toolkit for UI and Level design | Animation library

I have been making games in Libgdx and the only way that is use to make the game is through scene2d Stage. I make a BaseActor which extends Actor and make my actors and characters from BaseActor class.

But the thing is I wanted to make better games and use physics in it. I searched and searched finally I found box2d. I made a game in it using my own way again BaseActor, BaseBody,and Stage.

BaseBody for making bodies, BaseActor for making actors for stage and probably the same as my old ones but when playing it the gravity that I used can't move the objects faster. the movement is so slow. Then I again searched and searched and find that the problems is with metric system. I was using pixels and that made my game very bad. There was something I found on the internet with the name of ppm. I really don't have any idea how to use it or what it really is or any other thing related to it.

Then I tried to find a scene editor and found overlap2d and visEditor both goes down and not supported any more. And I want to say that I don't know the meaning of camera.combined, camera.setToOrtho and matrix. And finally when I made a game in box2d at least better than my old broken projects but still can't use metric system and a lot of bugs I saw that when the box with the image goes to the edges of the ground the body gets some kind of rotation but the image still is the same and I even use body.getangle and set image's rotation with that it still doesn't rotates. And I don't know how I should make things work again.

This is the game that I made using visEditor and works fine except setSize of the images and many more bugs about the origin x and y. I used scene loader of the viseditor in this and no Stage. And this use metric system.

And this is my game that I used stage and other things but no scene editor.

--- what is the meaning of matrix , camera.setToOrtho , camera.combined.

--- how to use metric system instead of pixels in both box2d and Stage.

--- is there any good, supported, and updated scene editor for libgdx.

--- is there any other way to make game instead of stage and actors but with classes and more efficient and code reusable for libgdx and box2d

--- how to apply the physics of the body in the image too.

--- am I coding right with using stage and actors and filling the body with image using the batch.draw() and using the body position and more or is there any method that fills body with image or any other at least better than my way.