I finished this review app tutorial where we used new theme app extensions to create app blocks for the dawn theme. The code for the extensions was built in liquid.
This may stem from a fundamental misunderstanding of how app blocks work, but could react be used to build the front end instead of liquid? For example, could this file which displays the average review rating be built with React? Thanks for any clarification!
The short answer is yes, of course you can inject your React into the theme using an App extension.
The problem is, you won't be offering any control of that to the merchant. Your React code does not play in the sandbox like Liquid does. So where some aspects of the Block are exposed to the merchant, which is a good thing, none of a React component would be.
If you want to play with React, Themes and Shopify, go with Hydrogen. If you want to stay inside the usual Shopify themes, like online store 2.0, you want to leverage the functionality of Liquid.
Anywho, I could wrong... have at er and see what happens.