Emoji
ctrl + ., but it only works under gtk application.
to adapt all application, u can install it.mijorus.smile and map a shortcut.
flatpak insatll it.mijorus.smile
flatpak run it.mijorus.smile
Unicode Method
Press ctrl + shift + u, then input the unicode.
Alternate Characters Key/Alt Gr
Alernate Character Key is a modifier key, ussally labeed Alt Gr, similar to shift. It asks the inpute source support the AltGr layout, then you can press Alternate Character Key and other Key to called a character which in AltGr layout.
Compose Key
more detial that you can see this.
Input Method
you can change the input method to get the special character you need.