SPC i u shows a pop-up list of Unicode characters by name, with each name also showing the unicode character.
The first time
SPC i u is called the unicode characters are loaded, as spacemacs doesn't load them in by default. Loading should only take a second.
As the popup list can be narrowed by typing part or all of the name of characters. The list shows a fuzzy match of unicode names that match.
SPC i u
|Loads unicode characters and lists them in a popup window. Uses helm completion
Classic Emacs Unicode 8λ︎
C-c 8 RET is the classic way to add Unicode 8 characters, which seems to show different symbols, is that unicode 8 and the other unicode 16 perhaps?