Ask HN: Best practices for onboarding new employees?

I'm starting a new gig as a product manager with a startup in a few days. ~20 employees (most but not all in one building). Good customer traction.

I'm building a personal checklist of onboarding items. It contains the usual categories - hr/payroll stuff, phone/email/slack setups, product details, customer commits/requests, risks/issues, etc.

The main concern is how to be productive from day 1 without being a time suck for the CEO & CTO. Does anybody have 1) a shareable best-practice checklist, or 2) a methodology of quizzing my new coworkers?

Appreciate it.