If i am correct, you can find the 2025 material folder at:
c:\ProgramData\SketchUp\SketchUp 2025\SketchUp\Materials\
As mentioned in the other post " Missing Materials in material pane 2016 Pro" you visited, maybe it was this “entire sketchup folder” that was somehow set to ‘hidden’:
c:\ProgramData\SketchUp\SketchUp 2025\