I just changed jobs after 8 years. I am one month into the new gig. What it did was spend the first two weeks solid, doing one-to-ones with all key stakes holders, and taking copious notes from each and every one. That includes engineering managers, team leads, architects, DevOps people, sales, legal, HR. Ask what's working well, what's not, top gripes. Talk to people who are resigning for exit interviews, lots of good inside info there. Then volunteer for crappy jobs to bid trust. Customer support escalations, documentation. This gives you a reason to dig and ask questions in a pointed way, and builds trust.