Changes to the screen resolution both in the launcher and in the dxcfg file do not lead to anything. Black bars on the sides of the screen still remain. What to do?

accept the fact they do nothing. cause they don't

DevilutionX allows you to play in widescreen mode:

https://github.com/diasurgical/devilutionX/releases

Thanks to! Now the image in the game is full screen. But I don’t understand, is this the original Diablo, or Hellfire? I couldn't find the entrance to the new dungeon, but difficulty levels appeared.

It can run either, you just have to provide it the MPQs from the game. It was always possible to start games in different difficulty, but it was just tricky to do in Diablo, DevilutionX makes it easy.

By the way there is a big update coming to DevilutionX later this month.

What is "MPQ" and where i can find it? And how i can provide it?

Please see the installation instructions: https://github.com/diasurgical/devilutionX/blob/master/docs/installing.md#installing

Thanks a lot!