Linux alternative to alt+numpad codes

I sometimes need to type "alt codes" to get symbols and on linux, it can be inefficient to use shift+ctrl+u then type the code, for example shift+ctrl+u+00a7 for § when on Windows it's alt+21. So what I'm asking is, is there any way to basically use alt+... for these symbols as I don't really like using shift+ctrl+u and it doesn't work in some places. I use xfce4 as my desktop environment and under the keyboard settings, I've seen something called a "compose key." As I didn't know what this was, I looked it up and saw it kind of had something to do with these unicode characters. I've also seen somethings along the lines of level 1, 2, 3 and 4 keys but I don't really understand how these work, so if someone could explain these as well, that would be great. (I use arch (manjaro more specifically) if this helps)