Hi @champtar,
they are stored as doc values internally and the best in-depth description I know about is in the Definitive Guide.
To quote specifically the part about strings (that you care about):
Strings are encoded [...] with the help of an ordinal table. The strings are de-duplicated and sorted into a table, assigned an ID, and then those ID’s are used as numeric doc values.
So strings are already stored as efficient as possible and there is no need for you to worry about encoding them differently.
Daniel