That last one is a biggie, as it'll let you install Magisk both without a custom recovery and without already being rooted. There are a few caveats.

Using the sideload command, and the newest version of adb from the Android SDK, one could update their Nexus device through stock recovery, without having to root or flash a custom recovery.

How to install custom themes without root Getting started: The prep work Download the Substratum application from the Google Play Store for free ...

If you’re a first-timer, then follow the guide to install TWRP on OnePlus 7 Pro and then root carefully. Note: Installing custom recovery will void your warranty.