When I run arc help land at rARC1b4a3e0c5eb9, the information it provides about what branch it will land onto by default seems to say that it will land onto master unless otherwise specified:
Land an accepted change (currently sitting in local feature branch branch) onto master and push it to the remote.
…
--onto master
Land feature branch onto a branch other than the default ('master' in git, 'default' in hg). You can change the default by setting 'arc.land.onto.default' with arc set-config or for the entire project in .arcconfig.
If I understand the change introduced for T5690: Respect tracked branches in arc land, however, arc land will actually default to landing in the tracking branch, if there is one. If that behavior hasn't since been changed, I think it'd be nice to mention it in the help text, so that people know that they don't have to explicitly specify --onto when they want to land their change in the tracked branch.