The older the system/the more changes it's had, especially if it's "enterprise software", the more insane things tend to be; in one memorable case, it was a .zip containing JSON, which then contained base64-encoded XML, wrapping another base64-encoded .zip containing more XML, and in that XML was base16-encoded ASN.1 PER with the actual payload (encoded in a proprietary format). I won't disclose specifics, but perhaps someone may recognise that from its description; suffice to say the industry that was related to started using computers in the days of mainframes and punched cards.

for games its more often .net BinarySerializer.

A lot of unity games does it, especially the 'legacy' serializer

I've not done unity3d dev- but yea, all I've seen from it are games using XML, or BinarySerializer. sometimes its compressed.

A lot of older stuff just dumped structs in general

Uhm... a (g)zip - yes, but a sqlitedb inside - no, not that often, in my experience.