Fascinating article by Emojipedia tracing the evolution of the first 12x12 and 16x16 emojies back to early Sharp PDAs, pagers, and Wapro electronic typewriter in the late 80s ...
Custom emoji are added to Mastodon etc by a server's admin. Anyone can see them from other servers, but only people on that server can use them.
For example :amiga: is a custom emoji.
To see your server's custom emoji, click the 🙂 icon. (On mobile apps this is within the app itself, standard emoji are accessed via the on-screen keyboard.)
Any image or animation can be a custom emoji, ask your server's admin if you have a suggestion.
@FediTips I've been on a few Masto instances where the Admins were kind enough to agree to add my preferred #FOSS#emojis. Since coming to #Friendica, unfortunately it seems like there's not a way for Admins to also add these emojis here, which is sad. 😥