After searching the internet, to input unicode, there is counsel-unicode-char (SPC i u). However, it only autocomplete on the name of the unicode, but not the hex, even though the UI suggested it should. I cannot find further information about that, so I need help for the issue.
Autocompletion on the code point hex wouldn’t make much sense, since characters sharing code point hex digits likely aren’t related (unlike characters sharing a word in their names). It would just pollute the completion, e.g. typing in BAB would show BABY-related characters but also whatever random characters happen to have BAB in the code point hex. If you know the code point, you already know exactly what to type.
Even though no completion candidates appear, typing in the code point followed by RET will insert the corresponding character.