What are the fundamental differences between a PMM and a senior PMM? What are some clear indicators that someone is ready to take on a senior PMM role and the responsibilities that come with it?

For me, it was primarily just based on experience, and having done a number of launches, GTM planning, messaging, buyer/market analysis, etc - that I know what to look out for, and don't need a lot of hand holding. You can read about how to do our work, but until you are in the trenches and learn what really works, what doesn't, and what to look out for, it's hard to understand what really goes into it. As a PMM I can hit the ground running in nearly any company, because I know what's expected, I know what has the highest impact the quickest, and I need very little guidance/direction. I've also learned what "red flags" to look for, having gone through the growing pains, and how to spot/correct them or avoid them all together.

My work is probably less scrutinized than a PMM who may be just getting started (or thinking back to when I first started). I'm not learning as much about PMM itself, as much as I'm perfecting my craft and skills in the field. I'm still learning of course, but it's how to go deeper, and how to scale the work that I do.  

I'm not sure it's really any MORE responsibility, in terms of tactically doing different things. I don't do anything more than a non-senior PMM does, I just may be more efficient at them and need very little direction on a path forward. I also know what things to focus on, track, measure, etc - those I think are really tough for people to grasp early on. If you have an eye on how the work you're doing is influencing the product's success throughout the year, then you're ready.