How can I configure shortcuts to special symbols or emojis in El Capitan or later I would like to input faster some special characters (math symbols, emojis, etc.).

I found this article, but the instructions are obsolete for EL Capitan or later.

I would like to add either keyboard combination shortcuts or a long-key-press popup (like for diacritics.)

Any ideas how to achieve one of the two ways?
You can make shortcuts via system preferences/keyboard/text/replace with

You can make a custom keyboard layout with

http://scripts.sil.org/ukelele