In a project team for a bigger project people have several roles:
The project lead can delegate parts of the project to a developer, who then takes responsibility for that part and takes over part of the project leads job in that regard.
In bigger teams, the project lead should try to assign topics to developers in such a way to avoid that unnecessarily many people have to be familiar with a specific topic.
If we have a dedicated internal PM, we should still treat them like an external PM. They are responsible for all PM work, but not primarily responsible for budgets, dead lines etc. This responsibility always lies with the project lead.