Debug react source code on yourself custom builds with source map. First of all, read offical guide How to Contribute and be familiar with it's build flow.
Clone the repository, use specified branch or tag, eg: v16.8.0:
$ git clone --depth 1 --branch v16.8.0 [repo url]
$ cd ./react
$ yarn
...
AssertionError [ERR_ASSERTION]: Current node version is not supported for development