This is something I have experience doing since I was one of the first monetization product marketers at Instagram and the first product marketer at Slack. I'll say that it was different at both companies, so it can vary. It really matters what your company's priorities are and what each individual is focused on and where their strengths lie.
PMMs do not support PMs 1:1 in small companies (wouldn't that be nice!), so it's best to be strategic about where a PMM can help fill in gaps early on. To me, this is really at three phases: 1) defining the MVP (through an understanding of customer needs, competitive overview, and market sizing), 2) determining if an alpha/beta is needed and managing those, and 3) launch and post-launch support.
I also believe strongly that a PMM should help define success metrics. If it's too early to set concrete KPIs, it's important to start building the foundation for what you'll measure and why.