This character is a Other Letter and is mainly used in the Arabic script.
The glyph is not a composition. It has no designated width in East Asian texts. In bidirectional text it is written as Arabic letter from right to left. When changing direction it is not mirrored. The word that U+0639 forms with similar adjacent characters prevents a line break inside it. The glyph can be confused with 28 other glyphs.
The Wikipedia has the following information about this codepoint:
Ayin (also ayn or ain; transliterated ⟨ʿ⟩) is the sixteenth letter of the Semitic scripts, including Arabic ʿaynع, Aramaic ʿē 𐡏, Hebrew ʿayinע, Phoenician ʿayin 𐤏, and Syriac ʿē ܥ (where it is sixteenth in abjadi order only).
The letter represents a voiced pharyngeal fricative (/ʕ/) or a similarly articulated consonant. In some Semitic languages and dialects, the phonetic value of the letter has changed, or the phoneme has been lost altogether (thus, in the revived Modern Hebrew it is reduced to a glottal stop or is omitted entirely in part due to European influence).
The Phoenician letter is the origin of the Greek, Latin and Cyrillic letters O, O and O.
The Arabic character is the origin of the Latin-script letter Ƹ.