Magisk is a highly advanced way of rooting android systemless-ly. This means that Magisk root android without changing or modifying the system partition. Hence you can receive OTA updates, run apps that require to pass Google’s SafetyNet tests.
I'm not sure I understand what you're saying. Are you telling me that you can simply copy the file onto an unrooted phone, and voila, you can gain root access?? Can you point to information about what to do and how this works? It runs contrary to everything I've ever read on the subject.
adb shell "/sdcard/su -c '<command-here>'"
This really seems contrary to everything I've read, and this Stack Exchange thread specifically explains why that wouldn't work, but if you say you have experience with this and it works for you, I'm certainly willing to give it a try and see how far it gets me. Do you know where a copy of the su binary can be obtained? All my searches for su binary lead to the supersu APK and instructions for installing it by flashing, or something along those lines. I can't find an su executable that can just be copied to internal storage as-is anywhere.I think I clearly expressed that neither a Custom Revovery nor Magisk itself is needed to have root, that simply copying SU-binary to Android's user-space is enough.
If you want to root via Magisk then do it.
Personally never would do it this way.