I’m not sure what the correct way to add them in is (newbie Q). I feel like someone has a better way to add them in. It just needs to extract the northeast Mass. area to put those bus routes into the map.
Well, first step would be to add the bus stops and then the routes. The data is from 2020 and split into stops and routes, which is not that useful. I wonder if you can get the original GTFS and use it inn OSM.
Anyway, I fear it is a task for little more experienced users and I recommend to use JOSM and its pt_assistant plugin.
For QA and a nice display of GTFS there is the wonderful tool called PTNA.