IIRC Apple originally invested in ARM during the development of the Newton. The original Newtons used ARM 610 CPUs. I don't know exactly when they sold their ARM stake but they kept their architecture license.
The Newton was long before the iPaq, the MessagePad was released in 1993.
On selling of the ARM stake - asked ChatGPT:
Q> And latter - was it that it looked that Apple maybe in danger of going under, and then they sold their ARM stake and got a cash injection that way?
A> And yes. In the late-1990s turnaround, Apple sold down its ARM stake in multiple tranches after ARM’s 1998 IPO, realizing hundreds of millions of dollars that helped shore up finances (alongside the well-known $150 million Microsoft deal in Aug 1997).
I remember iPaq PDA fondly. Wrote a demo to select a song from a playlist with few thousand author-album-song with voice query. The WiFi add-on was a big plastic "sleeve", that the iPaq slid into, not the other way around. Could run the ASR engine for about whole 10 mins before it drained the battery flat, haha. :-)