Here are some useful links:

• My goods on Asset Store
• "Real Materials" forum thread
• "Real Models" forum thread
• Advanced version of "Real Materials" for Blender (Cycles)

Important notice: make sure your system disk has at least 25GB of free space when installing "Real Materials" from Unity Asset Store.

1. Since two new render pipelines were added (HDRP and Universal) on top of Standard (Build-in) and lack of support of separate downloads for pipeline-specific assets in Asset Store there is a potential place for a problem with free space. Summary all 8 volumes of "Real Materials" series have a size of 25GB (!) because each volume contains 3x of assets (for HDRP, URP and Built-in rendering pipelines) and all those 25GB of assets stored here:
   C:\Users\*your Windows user name here*\AppData\Roaming\Unity\Asset Store-5.x\ (for Mac it's here, for Linux it's here)
   Each asset you ever downloaded from the Store has a copy in this folder and it's never cleaned up automatically it seems. So, if you have troubles with downloading or unpacking "Real Materials" files in Editor - delete everything from that folder, check if there are at least 25GB free disk space on system drive and try again.

Useful tips for metal materials usage:

1. Rich & contrast environment reflections are the key for the good look of the metal surfaces. Example - same material, same mesh and only environment reflections are different:
   
   
   
   
2. Strong highlights from light sources are needed for proper metal look&feel:
   
   
   
   
3. Bare metals looks poor on large flat surfaces. Bends and other geometry details are needed to improve situation:
   
   
   
   
4. Colored metals (brass, copper, gold) can't reflect blue color and become dark. Example of this with overcast sky VS default clear blue sky (on both screenshots it's same material on the same mesh):
   
   
   

Minimal supported Unity version for "Real Materials" series is 2019.4LTS.

To use "Real Materials" with previous versions of Unity first you need to create empty project using Unity 2019.4LTS and import needed "Real Materials" package properly. Then close Unity, copy "Real Materials" folder from there to your old Unity project and pray.