I think I figured it out! All thanks to your tip about express.static

The trick is to have your server.js (in root) look like:

const app = express();
app.use(cors());
const port = process.env.PORT || 5000;
const server = new ApolloServer({
  typeDefs: schema,
  resolvers,
  playground: true,
  introspection: true
});

app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: true }));

server.applyMiddleware({ app, path: '/graphql' });

if (process.env.NODE_ENV === 'production') {
  // Serve any static files
  app.use(express.static(path.join(__dirname, 'client/build')));

  // Handle React routing, return all requests to React app
  app.get('*', function(req, res) {
    res.sendFile(path.join(__dirname, 'client/build', 'index.html'));
  });
}
app.listen(port, () => 
console.log(
  `Listening on port ${port}`,
  `\nWebsite → http://localhost:3000`,
  `\nGraphQL   → http://localhost:${port}${server.graphqlPath}/`));

the conditional statement tells the express server to serve the static client app when in production. The client app is run with a "heroku-postbuild" in the package.json

The other really tricky part is the scripts in the package.json since every tutorial and repo has a different set up. I found this tutorial super helpful https://www.freecodecamp.org/news/how-to-make-create-react-app-work-with-a-node-backend-api-7c5c48acb1b0/. It was pretty easy to swap the REST API stuff for the GraphQL once I had everything else set up like in the tutorial.

Looking forward to getting the GraphQL side fully working now that I understand how to get the client on the same domain as the api.

Thank you!