Do you have root in Android P 9 ?
I do following step :
1. Flash to global to 8.1 (using fastboot)
2. Update to Android 9 (OTA).
3. Go to bootloader and switch to slot b
4. Fastboot boot twrp.img
5. In twrp go to advanced => adb sideload.
6. Adb sideload magisk19.3.zip
7. Reboot to bootloader and switch back to slot a.
But after install magisk manager it's still say that no magisk install. Did I doing wrong ?