Assumption being, if you can't pass interviews with another team, you're not good enough to stay. That only works for large campuses and of course, getting another role is more indicative of your networking abilities than skillset and experience.
Less of an interview and more of an informal team-matching coffee chat. They all have access to your co-workers and body of work, so no need to haze you with leetcode again.
That’s going away due to d&i requirements. They want everyone to have to apply for all open positions so internal transfers aren’t given preference over external.
Meta doesn’t do interviews when passing between teams though (at least not in the common case).