Audio Equalizer for YouTube Music app I want to adjust music played by YouTube Music app (don't mix up with Google Play Music).
How can I do it?

Built-in MusicFX equalizer doesn't affect YouTube Music. I also tried several external apps, which claim that they are OS wide, and in fact they're able to adjust sounds from standard Music app and Play Music app. But none of them work for YouTube Music app. They only can change volume while it's playing.

If you suggest app-based solutions, please, don't suggest apps that you haven't tried yourselves so they worked for you. If there's a solution with features shipping with Android itself, all the better.
I use Viper4Android and it works for me on YouTube (Have been using it since ICS era). It requires root though. 

Installation method various depending on your Android version.
If your device is supported by Magisk, I recommend you to use the Magisk Manager and install the Viper4Android module through it. 
Official way is to install using the app Official XDA thread (You'll probably need to set Selinux mode to permissive to let it install drivers which I personally don't recommend, so go for the Magisk way)