Much less structured than that. Make improvement part of the workflow. Much of this comes from an overriding principle of ownership where a team is given a business capability to own, not a project to deliver. They own how they solve the problems and they own the outcomes of their solutions. If I write buggy or overcomplicated solutions, I feel that when I get woken up to fix things. Teams with ownership will keep improving things as they go. In some cases, that permission to improve may require management explicitly stating it.

Project teams or teams under stress will not make things suck less daily. Retrospectives won't fix that.