NextJS Lite Example With React
You can also include React on the page if you like, but without NextJS client side code.
If you are using the approach espoused by this example project, you'd only need to include React on very few pages, and in general never send down any NextJS or React code. It's likely that the approach on the Index Page of using the API to render React is better in most cases, where it's a matter of only rendering something once, but in the case where you need a highly interactive component, this is a feasible solution.
Of course, you could load any client side framework you like, such as Preact, Vue etc and use that too.
Click to render some React here in the browser