wiktionary says that jej and jego are indeclinable and I cannot find a declension table elsewhere. Can someone please explain why cases are listed?
I think those are English cases listed, not Polish.
Probably some kind of template used for every noun/adjective etc. just for consistency and to show that jego/jej can be used in different cases.
I apologise if this is explained - I'm on the mobile app so I don't see any grammar notes. Jego pies is in the genitive, isn't it? Are we using the genitive because it's a possessive?
In this case, "jego" is the possessive adjective form of "on". Unlike 1st and 2nd person, 3rd person possessive adjectives can't be declined, so there's no case at all here.