The glyph is not a composition. Its width in East Asian texts is determined by its context. It can be displayed wide or narrow. In bidirectional text it is written from left to right. When changing direction it is not mirrored. The word that U+0398 forms with similar adjacent characters prevents a line break inside it. The glyph can be confused with one other glyph.
The Wikipedia has the following information about this codepoint:
Theta (UK: , US: ; uppercase Θ or ϴ; lowercase θ or ϑ; Ancient Greek: θῆταthē̂ta[tʰɛ̂ːta]; Modern: θήταthī́ta[ˈθita]) is the eighth letter of the Greek alphabet, derived from the Phoenician letter Teth . In the system of Greek numerals, it has a value of 9.