How to remove the blue background in the game display and so that it shows the game normally?

Question

Unity

Hello, I am new to unity and I have such a problem that the scene looks like on the first screen:
1)
Later, when I want to see what I got in game, it looks like this when it is turned off:
2)
3) But, when the game plays, looks like this:


The essence of the problem is that I do not know how to fix it.
1) How to remove the black frame?
2) How to make sure that there is no blue background during the game and how to move the design so that everything is visible at all resolutions, and not like on the screen ....

That is, there is something that needs to be done so that the entire image is shown on the screen , without that blue background?
Please, help!

Answer 1

A) Stretch walls / floors to the desired size so that they cover the entire screen;
B) Bring the camera closer so that there are no voids.
It is natural to expect that screens are from 4:3 to 21:9