In my previous job we worked like this. We had one dev branch that everyone pushed directly to and the dev branch was eventually branched to an RC branch which in turn was merged to master once the release was complete.

The team was small, around 6 people, and the codebase was maybe medium sized (~500k LOC). There was no formal review process, instead it was up to each team member to ensure the quality of their own and others code. In practice I would read through all commits that came in the previous day while having my morning coffee. If there was some egregious I would talk to whoever made to commit to make discuss if something should change, but this was fairly rare.

Formal PR reviews were only ever really used for new members or for bigger/sketchy changes where someone wanted more eyes on it.

Because I ended up reading most commits, I ended up knowing how pretty much the entire codebase worked. It takes a while for this to develop, but the more you do it the better you get at it, especially in the context of a single codebase.