You don't have to do it for any apps -- you can use their GUI volume controls. I've done it for Music because that's usually buried under a bunch of other app windows and it's quicker and easier to punch the Stream Deck button than to go searching -- I haven't done it for YouTube (yet) because if I'm watching a video it'll be frontmost and easy to adjust with the player's volume control.
If I did want to use a macro for YouTube it would be via the "Execute JavaScript in Browser" action, using something like
var vid = document.querySelector('.video-stream');
vid.volume = vid.volume - 0.1;
...but I'll defer to the Forum's JavaScripters on that one!
If you are constantly fiddling with app volumes anyway, you've probably already got macros to do that -- just use those.
And if you really want to go crazy with individual app volume control, look at @johns's suggestion of SoundSource over in this thread.