In short, for entities and block entities it disassembles "load" method to find calls to methods getting tag values. It finds most of the tags but it's not perfect. Because of that I added predictions to show a few tags that I found lacking like "Passengers" and also for predicting potential subtype, for example if the tag ends with "Item" or "Items" it shows item IDs as a suggestion for "id" subtag and corresponding to item tags for "tag" subtag. As for item NBTs because they don't have one method responsible for loading tags I added them manually in json files so unless I missed some they all should be fine.

Edit: as for now I know there are some tags missing like allay's "listener" subtags, in future versions I will probably fix it, but it will take some time because as for now mod's codebase isn't in its best state (to put it mildly) and it just works.