Start with understanding what business problem the app solves.

Understand roles in application (admin, user, etc.) Click through the application, do list of all URLs/pages, describe what you think it does and consult with PO/team to clarify understanding.

Then learn where data comes in/out and in what format. Study schema of database (hint: ERD diagrams). Then study business API (schema, endpoints, graphiql).

Then you can move to frontend. Frontend is easy after you understand all above :P You will basically know what it does/should do, so you don't even need to read code.

React code is very modular so it's easy to navigate, to find where to start simply use React Devtools (shows component name on hover), then you should be able to navigate rest of code via IDE/editor.

If codebase is of high quality it should also have React Styleguidist or Storybook with most components to play with.

To understand react it's best to write it yourself:

  * https://jasonformat.com/wtf-is-jsx/
  * https://dev.to/ameerthehacker/build-your-own-react-in-90-lines-of-javascript-1je2
  * https://pomb.us/build-your-own-react/