Can vary a lot from project to project. Usually there is a bottleneck where new code is certainly getting looked at before being merged, not that things can’t go unnoticed. Depending on the size of the project, full audits can be performed by third parties. If it’s popular enough or there are bug bounties up, random people might be looking for issues as well. In general, the less popular, the less likely it is someone has recently taken a look at the code.
Yeah more like safety in numbers than reading every line of code you run, which is impractical and only warranted for the most extreme threat models.
I don’t think plugin devs add such features too often. More likely will focus only on their functionality. Plugins are better avoided if you are concerned. They are often abandoned and possibly bound to weak auth systems as compared to the main program source channel. The advantage is their code is usually much much shorter and easier to check out yourself.