The legwork, thinking, and writing come from the software engineers who are also going to be building and maintaining the thing. The review simply verifies that their plan is reasonable and perhaps suggests some angles they hadn't thought about. Reviewers are engaged in the same kind of work as the people going up for review - which is how they are able to make useful suggestions. After the meeting, some will be going back to coding, others to investigate the alerts they just got paged for, others to design/UXR workshops, others to go run sprint planning, etc.

I would expect an Architect team to deal directly with the product requirements, stakeholders, etc. and produce a design that they hand off to others to execute, while they move on to the next set of requirements. Architecture is part of the project lifecycle for everyone. This process is simply the architecture complement of code review, only slightly more formal.