In practice, your branches come from three places: the follow-up questions a reader has right after the main one, the objections or hesitations that come up before someone buys, and the adjacent topics that give a decision real context. Together, they build out the page without pulling focus off the trunk.
For a wedding photographer whose trunk is "wedding day photography," the branches might be timeline planning, how coverage hours work, what a gallery delivery looks like, destination-wedding considerations, and family group shots. For a bookkeeper whose trunk is "monthly bookkeeping for small businesses," the branches might be catch-up bookkeeping, how a monthly close works, what software integrations are supported, and when it's time to add a fractional CFO.
The hardest part is knowing what not to make a branch. A topic that needs a full page of its own is not a branch, it is a new trunk. A topic that only a few readers care about belongs in an FAQ. The art of branches is deciding what belongs on this page, right next to this trunk, and what belongs somewhere else.