Why should I add it to the main default?
A folder has been determined for you to put the files belonging to Sketchup program. Here it keeps the file temporarily. If you accidentally delete it from favorites, you will have to do this again. If there is a problem with My Files on the “C disk”, if you show your folder on the “D disk” and change the folder name there after a certain point, the file will not be used because the path name has changed. That’s why I use it this way so that I don’t have to deal with it again and I can see and use my color palettes directly, and I keep my file archive up to date in the permanent “D” file.
To use your archive in the main material folder, click show hidden files after entering local disk. then follow the file path I wrote. You can add your material archive here.
Main default Material Folder C:\ProgramData\SketchUp\SketchUp 2019\SketchUp\Materials