Often times the interesting teams knew who they wanted to fill headcount with. They would say "stop by for an informal chat", then in that chat they would interview you on (e.g.) very niche terminology. After that they would tell you it is not a good fit. Tried to go to 3 different teams on my way out of Google and none of them were interested. I think it is a bit of a status game, like they are looking for a PhD or to justify a visa.

Specific examples, an Android static analysis team and Fuchsia security both passed after informal chats (unprepared interviews). I've spent a ton of time in reverse engineering frameworks, malware, and building automated code analysis solutions (with tons of bugs found to my name). When you have that experience, and they bring you on to do front end dev on some internal tool, like there is just such a disconnect.

At the time (2009 or so) it was hard to leave SRE and be a SWE because SRE had a hard time keeping employees given the oncall and nature of the role. My mistake was to tell people it was easy to leave SRE, which the head of SRE didn't like. He called my new manager and chewed him out. To his credit, my new manager told me I wasn't in trouble, but to be more circumspect when dealing with predatory leadership.

No, you can move easily, which is why he could piss off his current leadership without consequence.

Being able to move doesn't mean your current manager will be happy about you moving. The "easy" part of the process means that they just can't do much to sabotage you or your future.

It's varied. Some posts below describe the standard software test engineering. Test engineers on Google Fiber would buy every microwave and 2.4GHz cordless phone and baby monitor, and see if our changes to interference mitigation algorithms improved or regressed between releases. So you're basically in a lab trying to break Wifi algorithms, probably not writing much code. (Also things like "does our change to move iPhone 6 to 5GHz when it's closer to the 5GHz access point also work with and iPhone 5?")

This was a long time ago and it was a "bespoke" position created by the SRE team. I set up a continuous build and then fixed bugs until it went green.

Test engineers at Google at the time (~2008) were expected to build test infrastructure, rather than writing unit tests (SWEs were expected to write unit tests and integration tests), or to build complex system tests.