There seems to be a user experience miss here. For some reason, bus tickets are another tab in the search box grouped with Train Tickets.

There should be a category of Buses in the navigation at the top of the page next to Buses.

It was only after I intentionally compared OTA for breadth of service offerings that I didn't see a (long-distance) buses category. From my observation, the only tourism offering Trip.com doesn't offer (for now?) is the Cruises category.

The top categories according to Meltwater group are as follows: