Within the process, the committees are open to ask any question about anything. There's no restriction on subject matter.
In relation to the estimates specifically, the formal function of the committee is to receive the votes that are referred to this committee. So in the estimates process and in the supply process, the committee's report back to the chamber concerns only the specific votes in the appropriation bill, but that doesn't stop you from asking questions about any of those other things.
If you're wondering about where you would see the reporting on the actual spending and where those authorities come from, those are not just under statutory authorities but are a separate kind of statutory authority. In the case of Trans Mountain, it's under statutory authority that's not even reported in the estimates at all. It's the Canada Account.