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+0423 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:
U (У у; italics: У у) is a letter of the Cyrillic script. It commonly represents the close back rounded vowel /u/, somewhat like the pronunciation of ⟨oo⟩ in "boot" or "rule". The forms of the Cyrillic letter U are similar to the lowercase of the Latin letter Y (Y y; Y y), with the lowercase Cyrillic letter U's form being identical to that of small Latin letter Y.