These types of glyph substitution are easily handled by the context of the character with no other authoring input involved.
Authors may also use special-purpose characters such as joiners and non-joiners to force an alternate form of glyph where it would not otherwise appear.
For other glyph substitution, the author's intent may need to be encoded with the text and cannot be determined contextually.
This is the case with character/glyphs referred to as gaiji, where different glyphs are used for the same character either historically or for ideographs for family names.
[4] As of Unicode version 16.0, standardized variation sequences specifically for emoji/text presentation are defined for base characters in twenty blocks:[1] Other standardized variation sequences are formed with base characters in the following fourteen blocks:[1] As of 13 September 2022[update], ideographic variation sequences are defined for base characters in nine blocks:[2][3]