What's wrong with my node setup for PBR materials? I made this node setup:

To create realistic PBR rocky ground but with displacement added it looks like this:

What's wrong with my node setup and how to do it right?
I am using diffuse, normal map, roughness map and height map for displacement modifier. Allso I would like to know how can I hook displacement directly in node setup and I don't know the use of 2 first nodes in my setup (just saw them online always being attached to image texture nodes.
So
1)What's wrong with my nodes?
2)How to hook displacement into nodes?
3)Whats the purpose of Texture coordinates and mapping?

Sorry If I am asking basic stuff because I tried searching online but everywhere I find diffrent things with wierd setups full of math nodes and I don't understand witch one to trust more.
I am confused so I asked here, If this is possible duplicate please point out
Thanx!
1) What's wrong with my nodes?

There is no real reason to solely use a Diffuse shader. Virtually all materials have gloss, so you should either mix a diffuse and a gloss or use Principled BSDF.

2) How to hook displacement into nodes?

Turn on the Experimental feature set (can be found under the "Render" button). Go to your material settings and under "Settings" change from Bump to True. Then plug in your displacement map (you may need some math nodes to get a good looking result). Also consider using adaptive subdivision to ensure you have enough geometry to displace.

3) What is the purpose of Texture Coordinates and Mapping?

The texture coordinates tell your material what coordinates your textures should use—generated coordinates, the UV map, etc. The mapping node can be used to scale, rotate, or move your texture around independent of the texture coordinates.