@Sliver_Horizon, unfortunately there is no step-by-step reference to port Django pages to React apps, and there’s no single PR, either: most pages have been ported as part of overarching projects, each with their own requirements. Take a peek at the board of a current Django port under way: the Instructor Dashboard.
That said, if you were to start porting the bookmarks feature (which would be awesome), my recommendation would be to start familiarizing yourself with the frontend-app-learning codebase, as this is where the page would likely go. I’d also look into what building blocks we use to develop modern frontend features in the Frontend Technology Selection section of OEP-0067: frontend-app-learning is one of the oldest React SPAs we have, and not everything in it is exactly up-to-date.
It shouldn’t be rocket science: it looks scary at first, but the Learning MFE is just a regular React app, for the most part.
Otherwise, you will have to understand how the feature works from the edx-platform side, and figure out how to expose new APIs to the frontend if they’re not already there. In other words, it’s likely you’ll have to modify the backend to make this work, too.