rooted but it's all in vain, vain, vain!!

I think I finally managed to root my phone...

I had previously unlocked the bootloader (which I now read can't even be done on some phones - I need to remember to check that next time I'm buying a phone).
I had previously installed ADB and fastboot.

Now I installed the TWRP recovery, I think. Except that I selected the "Read Only" option at first, in order to make a backup of my phone before altering it. And after that, I can't figure out how to change it to non-read-only mode. But maybe it doesn't need to be changed; I don't know.

Then I used TWRP to install the Magisk zip.
Then I downloaded and installed the Magisk Manager apk.

After doing that, I think the phone is rooted.

But the whole reason I wanted to root my phone was to be able to increase the number of volume steps, via these instructions:
https://www.guidingtech.com/14998/customize-media-volume-loudness-android/

I downloaded the Baksmali & Smali jars... apparently there is no longer a bat file for running them, so I had to figure out the new commands.
But then, after disassembling the dex files, it turns out that the framework.jar / classes.dex no longer contains any "AudioService.smali" file. There is an "IAudioService.smali" file, but it doesn't contain any string starting with "0xft" nor with ".array-data 0x4". Nor do any of the other files from the disassembled dex file contain those strings.
So those instructions are of no help.

Now I found this: https://www.droidviews.com/increase-number-volume-steps-android/
But I'm not trying that today. I am fried, and I haven't even taken my shower yet and haven't really had my dinner yet, and I really ought to be going to bed very soon. AAAAAAAAAAAHHHHHHHHHHHHH