Being accepted at the conference is peer review. The process is often identical to submitting to a journal. Submit, get accept/reject/opportunity to resubmit with improvements, and if accepted, attend the conference, often giving a talk.
Conference organizers constantly have to recruit reviewers for this purpose.
Proceedings include the accepted papers.
And you are just wrong about the weight / prestige. It varies by field. In biology, conferences are much less prestigious. In engineering, sometimes conferences are the only thing anyone ever cares about and submits papers to. And there are many cases that blend the two, such as ACM, where conference papers can go into journal issues as well, and this spans many fields.