Maybe this isn't quite what you are asking for, but after overhauling the onboarding process for my last few gigs, these are my broad takeaways:
1. There should be a clear, linear progression of items for all newhires to "hit", along with expected timelines for each item. No "fanning" until they are out of the onboarding woods.
2. Everything should be documented. The first item from #1 should be a landing page that outlines and links to every onboarding topic, list, or general reference point, in order, and grouped logically. This page should be the one and only place any newhire needs to go to figure out what they need to do, or what they need to know, about anything. Docs should also include team workflow, best-practices, and tips from previous newhires.
3. Checklists are better than lists. Everybody has a tendency to get lost.
4. To the extent possible, all access credentials and account registration should be centralized to a federated SSO account, and should be automatically handled with minimum input needed by the newhire. They should only have to set up their password and 2fa, and everything else should be automatically ready to go by the end of the first day. Again, this is the ideal, and the only place I actually experienced this was at Google. All HR B.S. should be done before the first day, or by lunch on the first day at the latest. Newhires should be able to hit the ground running.
5. Other commenters mentioned the importance of newhires to keep detailed notes. Notes about what seems confusing, or strange, or important things learned, or ideas for improvements. Every. Day....Every. Thing. New employees are an EXTREMELY valuable resource, simply because they are the only ones with a fresh, external perspective of everything going on. They should be keeping detailed notes, and managers should be following up with them and their observations/questions at least once a week.
6. Newhires should be assigned a "mentor" they can go to for questions about this or that, and that mentor should be one of the previous newhires that just got through the onboarding process. Not only will they be closer to each other in the "beginner" perspective, but this forces junior employees to get teaching/leadership experience very early on, forges stronger team bonds, reduces the likelihood of shy newhires from falling through the cracks, and helps distribute the handholding load normally bore by senior managers and employees that are probably overworked with more important stuff as it is. Even if all the mentor can do is link them to docs or point them to who better to ask, that is enough.
7. Newhires should be responsible for improving and updating the onboarding documentation, for the reasons outlined in #5. This should be one of their first projects.
8. Part of the documentation should also include a list of "go-to" people or teams for certain topics as well. Half the battle of getting one's bearings is knowing who to ask about this or that, and I've only ever seen this passed down from oral tradition.
9. For the more technically inclined, one should have automation scripts to get their workstation setup the way they like as quickly as possible. Here are mine as an example: https://github.com/thefunkjunky/gs-dotfiles/ . This would normally take me days to do manually.
10. There should be a reasonable and steady progression of tasks/projects for newhires to complete to get them accustomed to the job, the technology, and the general team workflow. One tiny practice project followed by "ok now here's the job, good luck" is unacceptable.
11. Not only should newhires be asking a lot of questions, but managers and other team members should be asking THEM questions as well. Make sure everybody on the team is getting familiar with each other and taking time to reach out. Most newhires will be shy, and it is up to everyone else to make the effort. Also, try to fit in a team social thing like beer fridays or something the week a newhire starts so they can get comfortable with the team outside of the context and pressure of work.
12. "11" is a weird place to stop, but I'm supposed to be working, so here's a "12" to make this list a nice even number. I suppose that's enough to chew on for now.