Unicode characters can either be encoded directly in the document or accessed by reference.

You need a unicode-aware editor to encode them directly and then you just type in the character you want.

Otherwise you use a character entity reference:

[66]    CharRef   ::=    '&#' [0-9]+ ';'
                       | '&#x' [0-9a-fA-F]+ ';'

An example, some mathematics:

∀α∈Γ(...)

∀α∈Γ(...)