Who on your marketing team writes product marketing content, e.g., ads, one-pagers, web pages, etc and who is involved in the approval process?

It really depends on the org. In the smaller teams I've worked on, it has been part of my role (the pm) to produce and write all content (ads, one-pagers, web copy, emails, blogs, etc.).

In other organizations, it's been split (this is my preference). I'll create the overall messaging doc and the product-specific content (web pages and one-pagers). The messaging doc will then be used by other members of the marketing team to develop the content they need to get the job done. I make myself available for feedback, edits, and questions, but my goal is to give them the tools and framework they need to get their job done without being a roadblock.