Materials doesn't appear- How I decrease the numbers of polys

You haven’t added a vertex group to the modifier - see how “Vertex Group” is empty, unlike my screenshots?

Hello, senior donmccurdy

How do I select and delete unused UV sets and vertex color sets?

I installed node js. How can I use that to re-weld vertices after export?

Hello, sir, josephhansen

As you see, your guidance did work, but the number of the meshes is still 18000, why?