What is the most effective way to onboard yourself to a new team's domain?

First, I recommend the other following discussions:

• "What’s the most effective way to switch domains in tech?"
• "How do I identify the core things to learn when picking up a new tech stack?"

I often feel intimidated with the sheer expanse of domain knowledge to be grasped...

I think Cat Chen had a great point in the Q&A about switching domains above: Embrace being a n00b, and just forget that you're a senior engineer.

There's no need to be intimidated. Just learn what you can one step at a time, ask great questions, treat your teammates well, and be hungry for knowledge. And of course, make sure you invest a lot into building up that strong relationship with your manager so they're a constant source of real feedback and you know where you stand (i.e. "Am I onboarding fast enough?").

I can totally see how it could be overwhelming to learn a new team's domain / vertical from scratch.

My thoughts are:

1. Get clarity of what's important moving forward and learn by solving a problem. A team may own a lot of things but not everything is important, focus on what's important. This requires proactive alignment with your manager / new hire mentor.
2. Build a learning agenda - following techniques from @alex chious's answers and build yourself a 30 - 60 learning agenda, carve out time for deliberate learning. Keep in mind that every company / team is different - some have strong documentation, some relies on tribal knowledge, some has good onboarding programs... observe what's effective in your environment to avoid wasting time.