[Magisk] pndwal:
Nah, info I gave is correct... But of course /recovery partition is needed to install Magisk in recovery ramdisk. 😉

All A-only devices have recovery partition and these all include a generic ramdisk!... But why would you install in /recovery (requires recovery key combo with attention to Magisk's release timing to boot to system with Magisk) when /boot includes working ramdisk or a manually added one is supported?....

So option is only available for those 2018, 2019 'type III' boot-type devices as I said...

The ZenFone 9 seems to be an Android 12 / 2022 device, so factory ramdisk in /boot = yes, and Recovery mode option won't even appear!!

The issue with these first SAR devices for John was simply that instead of using initramfs as rootdir and executing /init to boot (as in original "Legacy ramdisk" booting), kernel now directly mounts the system partition as rootdir (bypassing initramfs, ie ramdisk, altogether) so boot ramdisk was eliminated. However with A/B (Pixels from A7 after retrofitting SAR), since recovery ramdisk is moved into /boot, bootloader support for ramdisk in /boot necessarily remains and Magisk actually 'hijacks' that recovery-ramdisk to inject magiskinit and boot system with SU.

With A-only SAR however, there is NO factory ramdisk in /boot so bootloader support is usually dropped, so, unless bootloader supports a manually added ramdisk as in the case of Xiaomi and a few other OEMs, the ONLY option is to install Magisk in /recovery partition!... A-only 1st gen SAR devices are all launched with Android 9...

Nb. Magisk in recovery mode MUST be patched to recovery.img (which is also a boot image) using Magisk App; .zip installation from custom recovery cannot do this (always injects in /boot).

From Android 10, all devices have launched with "2SI ramdisk SAR" (a form of SAR, but generally distinguished from SAR by Google), and while final rootdir is still system, the kernel again (as with good old "Legacy ramdisk") uses initramfs (ramdisk) as rootdir but executes /init in rootfs to mount the system partition as the new rootdir before finally executing /system/bin/init to boot... This means that all devices launched with Android 10+, including all A-only devices with a recovery partition, again have the requisite ramdisk in /boot to support boot Magisk!... No longer is recovery mode Magisk needed nor does the option appear in 2SI devices...

Strangely, for A/B 2SI devices the recovery ramdisk in /boot partition is not just 'hijacked' by Magisk for booting system, but is used this way from the factory, and Google officially calls the method "USES_RECOVERY_AS_BOOT". So the recovery ramdisk has been hybridised to boot system in all modern A/B devices... For A-only 2SI devices (and some A/B devices with dedicated a/b_recovery partitions, notably OnePlus models) boot ramdisk(s) (akin to traditional "type I" boot ramdisk) has been restored to mount and boot /system...

Magisk has done wonders to keep up/work around Google's tangled Android boot architecture changes, and John's dev doc Android Booting Shenanigans is simply the best summary of these changes (as well as essential Magisk init injection method changes) I've seen... My summary above is largely an exercise in ripping off that! 😁 PW