PBR references to refer to for tutorial and explanation

https://blenderartists.org/forum/showthread.php?398687-Meet-Poliigon-the-next-logical-step-in-Texture-library-evolution

http://www.marmoset.co/toolbag/learn/pbr-practice

http://www.marmoset.co/toolbag/learn/pbr-theory

http://www.marmoset.co/toolbag/learn/pbr-practice

https://www.fxguide.com/featured/monsters-university-rendering-physically-based-monsters/

untiy specular vs mettalic
https://docs.unity3d.com/Manual/StandardShaderMetallicVsSpecular.html