Short answer: With a very small number of exceptions, we trust FreeBSD developers to exercise good judgement in obtaining review and ensuring the quality of the code they commit. The FreeBSD project is selective in whom it gives commit bits to, and we have a mentoring process which further ensures that developers understand the norms.

This system isn't foolproof, but it generally works very well. There have been discussions about moving to a "mandatory review" model, but there is concern that this might overly slow down development in the context of a volunteer-run project.