Which ones wouldn't you want to be on? Or at least is there a pattern to them?

This is purely my preference, but anything customer facing would be a snoozefest for me.

Most L6+ that I've met in infra are incredibly sharp, and there's tons of interesting problems being solved in infra/core systems right now.

On top of this... the "move fast" motto really does ring true, compared to google, facebook speed is like riding a bike vs walking.

Is it really true that once you pass the interview and orientation you get to choose the team you want to work on?

For general role hires, this is true, although it's a subset of all teams - teams with high priority roles pitch to you, you choose a couple to try out tasks for a few days. They stack rank, you stack rank, matches are made.

Internal transfers after 1 year are pretty easy. I've seen people transfer earlier, if it's clear the fit between employee and team isn't there.

      if it's clear the fit between employee and 
      team isn't there. 

This is very interesting to me. How good is management at discerning between...

a: "employee 12345 has been here for one year and probably wasn't a good hire"

and...

b: "employee 12345 has been here for one year, and they were probably a good hire, but they're not a good fit for their current team so let's get them to a new one!"

???

On a human level, I'm a firm believer that (b) happens a looooot. Personally, I've been anywhere from a turd to a star depending on company culture and team fit, and it wasn't me that was changing.

But I am amazed there are organizations that manage to spot (b) and actually correct it rather than cutting a person loose or just pidgeonholing them as a low performer.

Def seems hard to do. You could pass an A many times before ”proving” they aren’t going to fit in any team.

One way to side step the issue - trust the hiring process. I haven't seen very many issues, but Meta seems to give people multiple chances, on multiple teams, under multiple managers, before doing a pip (or whatever the equivalent is).

Yes, if they have headcount, you should be good to go