Craig Federighi SVP of Software at Apple “The keyboard now leverages a transformer language model, which is state of the art for word prediction, making autocorrect more accurate than ever.”[1]

"New features available with macOS Sonoma" doc states "Autocorrect fixes mistakes for you more accurately than ever before by leveraging a new transformer language model in English, French, and Spanish keyboards." [2]

"Introducing Apple’s On-Device and Server Foundation Models" blog post states "Apple Intelligence is comprised of multiple highly-capable generative models that...have been fine-tuned for user experiences such as writing and refining text.. In the following overview, we will detail how two of these models — a ~3 billion parameter on-device language model, and a larger server-based language model."[3]

"A look at Apple’s new Transformer-powered predictive text model" blog post[4] is the kind of deep dive you'd have to do to find out what's going on under the hood in your spelling/grammar checker. TL;DR AppleSpell uses GPT-2 which proves my point.

[1] https://eu.usatoday.com/story/tech/2023/06/06/wwdc-2023-appl...

[2] https://www.apple.com/mideast/macos/sonoma/pdf/en/macOS_All_...

[3] https://machinelearning.apple.com/research/introducing-apple...

[4] https://jackcook.com/2023/09/08/predictive-text.html