It's a quick read and an interesting peek behind the curtain at Apple during the development of the iPhone, with a focus on designing elements like the keyboard and predictive text.
What I took away from it is that the design decisions at Apple are in the hands of a few. The chain of command is small, it takes countless iterations until it "feels" right and you obviously can't consult with too many people when it's a secret project. A/B testing is out of the question.
And if they had tested the keyboards with any multilingual users they'd have found the new ones are broken. They overloaded the Fn key as the "switch language input" key so trying to do any Fn features like Fn+Right for End or Fn+Down for PgDn now has a 50% chance of changing languages.
(Straying a little off topic to provide the following hint:)
There's a setting for this in System Preferences > Keyboard > Keyboard. My recollection is that this was previously a checkbox that toggled the "double-Fn to invoke Siri" function on and off, but as of Big Sur (at least) this is now a popup menu control, where the choices are "Change Input Source", "Show Emoji & Symbols", "Start Dictation (Press Fn Twice)", and "Do Nothing".
My test M1 machine was set up from scratch last year (i.e. no account migration) and has "Do Nothing" selected here, so I can't confirm that "Change Input Source" is the default for new users, but its position as the first item on the menu is suggestive.
i use the caps lock key to swap keyboard languages. it’s a default option in the keys’ advanced settings, and it works wonderfully well (you can still invoke caps lock by long-pressing the key, too).
