A complex front-end code in your workplace should be similar. Hopefully a seasoned front-end developer is maintaining this complexity and if a back-end developer needs to contribute a trivial change, they can just `npm start` (documented in the README.md) which sets in place a whole lot of complexity that the back-end developer doesn’t need to understand, but just works.
Ideally you would have a pipeline building the project on each branch commit so that the backend dev can push, wait 5 min, fetch the artifacts for that branch and run it locally to test it. Or even better - if there's a pipeline chances are there's a Dockerfile doing the build but it will not support live-reload.
Either ways it gets complex as soon as you have to get out of your comfort zone. Especially if the docs are outdated/verbose/non-existent.