First 100 days in a job quite important. The First 100 days are your opportunity to ask questions, make some bold moves, build trusted relationships, and set the tone. I would focus on the following things:
Build a solid understanding of your industry and target market: As a PMM, you need to bring unique perspectives to the table. PMMs are fortunate that they get to interact with customers, sales, analysts, product managers and gain insights about competitors. In the first 30-60 days, I will focus on understanding things like market opportunity, competitive differentiation, why people buy us? or why they don't buy us?, TAM, Your core market, your adjacent markets. Trust me, invest your time in understanding these things better and you will get payoff in the next 12-18 months. So, although this will never be part of your initial projects, make sure you save time to build your own point of view o the market.
Think of your lack of knowledge for processes/tools more as a "fresh perspective" you can bring to the team: As a PMM leader, it is important that you invest in building scalable processes and you invest in building some foundational templates. So, while it's important to understand current processes/tools/SLAs, don't be afraid to propose new things that could save hours and hours of your team. People who are already following set processes tend to miss out on improvement opportunities. You can add tremendous value here. One of the best pieces of advice I got was from a PMM leader at Salesforce. When I was moving to a new company, she cautioned me to stay away from saying that the things are really broken (in her words - that the kitchen sink is dirty). Be mindful that the team has been using/working/following their own processes that may not be the best, but they got work done.
Focus on building relationships: You have a blank slate. Use it wisely. Build solid relationships with sales, product leaders, CS leaders, your team, your managers, extended marketing org. Try to listen before jumping to conclusions. It's difficult to do it during COVID time but casual coffees and happy hours can forge good relationships with your colleagues.