Jonas,
Thanks, I've understood. I'll try tomorrow with target "Darwin" + aarch64 on iMac.
It's a bit strange that first Macbook can compile to aarch64 with target OS "By default" (which works as "darwin").
But another iMac wrongly interprets "By default" as "classic macOS" and cannot compile to darwin-aarch64.
Both Macs with Big Sur 11.0.1, Xcode 12.2, Lazarus 2.0.10 and FPC 3.2.1 (fixes).
Probably some wrong old settings in /etc/fpc.cfg ?