Magisk

TL;DR

This procedure works for a OnePlus 5 phone.

download the os' recovery-flashable zip file
extract the boot.img file from the zip file
open the Magisk app
press the Install button on the Magisk card
choose Select and Patch a File under Method
select the boot image; the Magisk app will patch the image to [Internal Storage]/Download/magisk_patched_<random strings>.img
copy the patched image to your computer using the file transfer mode or adb:
```
adb pull /sdcard/Download/magisk_patched_<random strings>.img
```
reboot the device to the bootloader (fastboot)

flash the modified boot image:

sudo fastboot flash boot path/to/modified/boot.img