please tell me how can I get rid of shading problems? In the first screenshot, I set the hard edges and used auto smooth, and in the second screenshot I used the Weighted Normal modifier. But in both cases there are problems. How can they be eliminated?
All shading problems like this can be fixed by adding more geometry so that less interpolation is left to the shader. From the name, I gather this is your "low poly" version of these assets so you want to avoid adding unnecessary geometry, so start with the closest (which you appear to be getting with the Weighted Normals) and add more geometry.
For the error on the top plate (not the screws, which is too small to guess at without wireframe), it looks like you just need to add an edge at each end closer to the corner such that the center is a planar quad.