Beginner

Here is a very good tutorial on how to texture pbr materials

Blender material nodes explained by Daniel Kraft

A good technique to figure out what a node does is to select a node and play with the parameters.

3 Different Quick Effective TEXTURING TECHNIQUES - Hunting Knife - Blender 2.8 using nodes

Intermediate

This person has 3 ways to add layers to blender textures.  The middle one seems most versatile allowing you to resize and move your decal. Also there is a free decal pack.

The beginning of this tutorial, he talks about textures, and why you would use certain nodes with examples, like Noise, Musgrave and ambient occlusion.

From blender foundation, how to use Procedural Textures.

He explains how to use this node. 

He explains the nodes, and values.  The part I liked was how he explains dimension very clearly.  Now I know how to use this shader.

He describes which nodes (noise, and voronoi) to use and why.  Even better, he adds nodes like the color ramp and says why to use that.  Makes a great scratches Tutorial.

Very good procedural nodes videos.  You learn a lot about what to use and groups.

 Texturing for Beginners: Top 14 Ways to Mix Textures and Shaders (in Blender) - YouTube

He lists several ways to Texture in blender. Very informative.  Important he goes over how to make

A mask, how to paint on the mask, and how to use that to add grunge etc.

He adds an Image texture as grunge by using the Texture coordinate with generated, mapping, , image texture with box mode, a color ramp to control the amount attached to the fac of  a mix rgb for color. Add a second uv map which is very useful for decals. Use uv node connect to image texture -- how to mix texture techniques (Important to easily place decals)

5 Must Know Tips With Bump/Normal Maps (Blender 2.8) • Creative Shrimp Tips on bumps and normal maps How to bake a normal map

Excellent tutorial showing how to animate a Glowing .Transformation effect. If you think of the shaders as masks, you can understand what he's doing in this video easier.  He uses the gradient node and a noise node as a mask for the transition.  He uses a color ramp to make the lighting effect at the edge.  The mask is controlled by an empty.

Advanced

This tutorial is very advanced, uses math, but he explains his ideas clearly and you can see how powerful using these nodes are.  These 11 tips sumerize what nodes do what, and best of all, he explains why he is using them. He has some examples of what you migh do with them.  He makes a node group out of his node tree.  

Top Blender Shader Tricks from Nodevember 2019 ft. Simon Thommes - YouTube    Skip this video until you have a good grasp of nodes.   These are advanced math stuff.  Very interesting to see what you could do with nodes. This tutorial is more of an idea type of video.

Highlighting edges for grudge maps and dirty maps - useful. --  texturing - adding realism

Worn edges node  tutorial.  Very interesting.adding realism on the edges.  There are several ways to do this.

Creating a rust texture using nodes

This Tutorial shares tips on what the difference is between unreal engine and blender in terms of rgb channels in a single file.  The separate rgb node is the way to access info for those materials packed that way.
He has great tips that arn't immediately obvious in blender.

Also remember to use a color ramp to manipulate the mask. Also he typed in #frame into one of the values which tells blender to change the value every frame. Randomizing the procedural image. He puts that "#frame" word into the w value of a noise node which is I think is time.

His explanation of the uv map, helps to figure out how to align the muzzle flash using gradient map, and color ramp and  He uses to different planes, one for the part near the gun, and a second map that's elongated for the front.