1. Connect turned on phone to PC
2. adb reboot bootloader
3. Wait for fastboot logo
4. fastboot boot boot.img
5. Should boot after about 30 seconds
It worked! Thank you BlackChimaera for your advice.
I have been trying to boot from boot.img/recovery.img with "fastboot boot" command but it doesn't work, so I can't root my phone with magisk.
I have to use "fastboot flash" to root it, and that way I lose the option to do OTA updates.