afaik web components are mostly about encapsulation and semantics; they let you box up arbitrary HTML, JS and CSS and put a little bow on top so that it looks like one “custom” HTML tag to the rest of the document. (I could be misremembering.)

React is mostly about life cycles, propagation, and composition. The first two are orthogonal to web components afaik. The third has some overlap, but it’s mostly complementary with web components (once again, afaik.)