I discovered the compose key yesterday when exploring Gnome Tweaks and then I found this resource, which explains how to enable it and lists many of the key combinations. The compose key allows you to type special characters on Linux by pressing the configured compose key followed by a sequence of, typically, two or three characters. It makes it a breeze to type an ellipses, em-dash, and a whole host of others. Very neat.

➝ Source: fsymbols.com