i know that codebase, and the associated btcd codebase very intimately.
it is like being in the room where the shit is hitting the fan trying to do anything with it. i mean literally, it will raise your blood pressure.
i wish i could clean it up because whoever is paying for its maintenance clearly is being ripped off, or not paying enough.
also, taproot is basically just a mechanism for addressing contract addresses. you have the root, which is a regular segwit address, and then you can tap into that with basically infinite sub-addresses.
some of the terminology and design and explanations of the things really leave something to be desired.