should the polygons of a model be squares/rectangles? I don't really know how to ask this question, I googled, but I couldn't explain myself in a very short line in the google search bar, so I didn't get any result, so I thought it would be better to ask it here.

I'm new in 3D and I'm teaching myself, and whenever I make a 3d object and compare it to a good model that has been made by a professional 3d artist, I always notice that the vers that I have in my model look terrible compare to them. by that, I mean that the verts I put in my model are not even, whereas all of the faces(i don't know if I'm saying it right or not) that a professional make, always looks like a square and have only 4 verts. they always fill 4 verts into a plane and make keep it look like a square or a squarish rectangle(if you will) but mine are not like that.
some times in my models I have a plane that has 7 verts, and it's not square, not even a rectangle.

I know it's difficult for you to understand what I'm saying, I'm just trying my best to explain.

Please take a look at these pictures, they will help you understand what I'm trying to say.

This is a model that I found in google.


If you take a quick look, the faces are almost in the same size, and they fade into smaller ones.
and they're all even and looking squarish.
(Yes I know it's in edge select mode which doesn't show verts) but I think you get what I mean, I have seen many models in vertex select mode and they're like that.

This is my model, one side of a bus


polygons are not even at all, sizes are totally random, and some of the polygons have more than 4 verts.

My questions are, Is it important? if yes, should I fix them one by one or is there a better way or an addon to do the job?

in addition to that, I'd like to kindly ask you to please, explain to me a bit more about these polygons. triangle polygons vs square polygons, or things like that, if there's any tip or important thing that I need to know about polygons, faces, verts when modeling.

and one last thing, I really thought about a good title for my question, but I couldn't find anything that can explain my problem, hope I'm not getting downvotes.

Best regards,

Amirreza
The term you are looking for is "quads".  Tris have 3 vertices and n-gons have more than 4.

Quads are better in general for modelling in Blender, particularly if you are planning to animate.

Quads allow for clean straight lines.  This is SUPER important.  Loop cuts can easily be introduced, selection is easier, UV unwrapping is easier, beveling can be done. 

The list goes on and on.

You will learn how to create quad based topology.  It's worth the effort and a great foundation for learning Blender.  So good for you!  You are learning one of the most important early lessons in modeling.