The short answer is that the UI isn’t optimized for users like you.

I haven’t worked for Google specifically, but at this scale everything gets tested and optimized. I would guess they know power users like you are frustrated, but they know you’ll figure it out anyway. So the UX is optimized for a simpler target audience and possibly even for simpler help documents, not to ensure power users can get things done as quickly as possible.

I feel like you're giving too much credit here. I don't know if it was a leak or an urban legend, but I remember the awful win 8 "flat boxes UI" being that way because it could be designed by managers in PowerPoint that way

The specific feature in question...there is nothing "power" about it. It was a non-feature for decades essentially, I dont recall ever not being able to simply change an e-mail address by moving the cursor and typing in something else. How on earth is this something tested and optimised, for whom exactly?

This is almost certainly not the case. The larger the company the more change is viewed as a negative. Yes people may hold titles to do the things you describe but none are empowered to make change.

Google UI seemingly is optimized for happy path cases. Search for the obvious word and click a relevant link on the screen which appears. Write a single response to a single email and abandon than conversation afterwards, always use new conversations for every new email. Click a recommended video thumbnail on the frontpage and then continue with autoplay. Put only short defined text type in the cells of a spreadsheet, like date/number/text etc. And so on with all of their products.

But as soon as user tries to search for something no on the first page, or reply to a 10-20+ message thread with attachments in history, or tries to use playlists or search in YT, or input a slightly more complex data in the sheet cells - then all hell breaks loose.

Just the latest Google thing I've experienced - a default system Watch Later playlist is now hidden on Android. It's gone, no traces, no way to search for it. The only remnant of it is a 2-second popup after adding a new video to Watch Later, you can press "view" and then see it. Meanwhile it is still present as a separate item on PC. I'm writing this eaxmple because that was deliberate, that was no error or regression. Someone created a Jira for that and someone resolved it.