One of the tricks I like to use to get up to speed quickly is making a hyperlinked dictionary / glossary (a wiki or a github page with anchor links work nicely). The biggest roadblock to understanding the code you read is not knowing the exact meaning of the terminology chosen by the original developers. Building the dictionary helps internalise it quickly, leading to much less confusion when reading.

Terms present in the UI (or API docs) of the segment you wish to understand are a good starting point.