Custom material library not working properly

Can anyone help me fix my materials library on Mac please?
I often make my own materials - every project i work on requires a particular wood, tile, brick or whatever. But sometime i want to re-use them on more than one project. Yes, i know I can open an old model and copy the object/block into the new model, and that brings the material library with it; but that requires me to go find the project that had the particular texture i am looking for.
I’ve watched YouTube videos but they are mostly PC only and this, i understand, is very different on Mac.
So in my current model there is a folder just after “Metal”, and just before “Patterns”, called “My Textures”. I can add materials to that folder, and that folder appears when i open a different SU model, but the textures associated with each material don’t seem to carry over…? Also, I often create a material by taking an existing brick (or whatever) and duplicating it, then changing the associated texture image. But that new material then appears in the SU Bricks list. Can I move it (save to) my own custom list? there is no save as, or Move to, and I don’t know where the libraries are kept!!!

You should create a folder with all the materials you work with, that way you wouldn’t have to find a project to copy the materials.

I’m not sure if it’s possible, I’ve never tried to find the materials folder in the sketchup folder, and insert the folder with your customs materials, you can try to do that, first press Shift+Cmd+.(dot) it will show the hidden folders, I don’t remember now where is the sketchup folder, maybe someone else knows, and also if it’s a good idea or if there’s an easier way to do it.

Yes, I have a folder of thousands materials built up over the years (starting when I used to do 3d in micro station!), all in jpeg or png format, saved and indexed. That’s the first place I go when I need to create a new texture in SU.
But when I do that, and use it, I want be able to maybe use it in the next model, but it’s gone.
Any more ideas please anyone?

I don’t know where it is on a Mac but on a PC, under the Preferences tab in the Windows menu the File heading has where materials and components are saved to