Oh, I made React work in Ember

controllers.application\.js

import Controller from '@ember/controller';

export default class ApplicationController extends Controller {
  appName = 'Ember Twiddle';
}

routes.application\.js

import Route from '@ember/routing/route';
import React from 'react';

const e = React.createElement;

 class Hello extends React.Component {
  render() {
    return e('div', null, `Hello ${this.props.toWhat}`);
  }
}

export default class Aplication extends Route {
  setupController() {
    ReactDOM.render(
      e(Hello, {toWhat: 'World'}, null),
      document.getElementById('react-root')
    );
  }
}

templates.application\.hbs

<h1>Welcome to {{this.appName}}</h1>
<br>
<br>
{{outlet}}
<br>
<br>
<div id="react-root"></div>

twiddle\.json

{
  "version": "0.17.1",
  "EmberENV": {
    "FEATURES": {},
    "_TEMPLATE_ONLY_GLIMMER_COMPONENTS": false,
    "_APPLICATION_TEMPLATE_WRAPPER": true,
    "_JQUERY_INTEGRATION": false
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.js",
    "ember": "3.18.1",
    "ember-template-compiler": "3.18.1",
    "ember-testing": "3.18.1",
    "react-dom": "17.0.1",
    "react": "17.0.1"
  },
  "addons": {
    "@glimmer/component": "1.0.0"
  }
}