Some of it is due to washing clothes made of nylon or polyester, and of course that water eventually will go into the ocean.

The other source aside from tires and those beads is all the other plastics that we use. Especially when exposed to the elements, all(?) plastics eventually break down into tiny particles. This includes all synthetic fibers, by the way. I’ve seen studies that show how much synthetic clothes release microplastics each time we wash them.