It's not based on protobuf. There's some in-house tooling that checks the schemas of producers against consumers against known versions (aka the versions in production), and we run it as part of the CI.

Since services implement standard components/protocols there are standardised hooks to extract these schemas definitions from each HTTP endpoint or Kafka consumer.

Does this rely on code generators or maybe a schema generator from code? I’m curious about how schema and code are kept in sync.

Yes. The schema definition is part of the code/data itself, so you can reference a particular schema by its git rev hash.