Can I use web components in react application?

Yes, you can use web components in a react application. Even though many developers won't use this combination, it may require especially if you are using third-party UI components that are written using Web Components.

For example, let us use Vaadin date picker web component as below,

import React, { Component } from 'react';
import './App.css';
import '@vaadin/vaadin-date-picker';
class App extends Component {
  render() {
    return (
      <div className="App">
        <vaadin-date-picker label="When were you born?"></vaadin-date-picker>
      </div>
    );
  }
}
export default App;