Polygons showing in preview (bad mesh) name for this? or how to fix? thx

[20023546]

Hey this post might be dumb but I have a problem that I've ran into multiple times over the years but never know what to search up or the word for this. My guess is theres something wrong with my mesh, triangles, too sharp of a angle etc etc, but how do I fix it, is that really what is happening? And what am i doing wrong that is causing it

This "Thing" that makes me see my polygons when I try to review my work. I always thought it was bad mesh but when I R(scale) then push it so it evens out sometimes it fixes, sometimes it stays. 

You can see that it shows triangles on the top left, but when I get rid of the triangle it stays, I don't know why.

 

When I click smoothing mesh sculpting tool, it shows it in a different colour.^

So I've got to a point where I'm making my first (HELP ME) post.

In summary could someone please answer:

Why this is happening?

What I should do to prevent this (am I modelling / retopologizing wrong?)

How do I fix it, not just on this gun but anytime I see this issue.

And what do you call this? If I were to search up how to get rid of (xYZ)

And yes I've tried smoothing the edges, scaling it to fit/flow better, removed triangles and so on.

Thank you very much for your time and sorry if this seems ridiculous and simple.

[damaggio]

Unlock your normals and apply a new default Normals , the dark mesh means you might have flipped face normals.

Overtime with lots of practice you will improve your topology.

[20023546]

This isn't flipped normals/faces unfortuntely, it would be much darker and reversable.

my vertex and face normal's are facing the right way, the problem still occurs. Please help cheers!

[damaggio]

You have 3 different lambert shaders but without the scene file there isn't much more I can do to help you with the modeling process.Zip the file before attaching it here.