Your up dry searches are surely correct. Apple does not currently ship it's own RDBMS.
What Apple does provide, however, is Core Data - quoting off the 'net: "...an object graph and persistence framework provided by Apple in the macOS and iOS operating systems. It allows data organized by the relational entity–attribute model to be serialized into XML, binary, or SQLite stores."
CD requires multiple non-trivial skillsets, so be careful when making assumptions about applying it for your needs, I think.
See 'What Is Core Data?'
I need to develop applications that work with relational databases such as Microsoft's SQL …
Can you post more information about your goạl here? The reason I ask is that there are lots of different approaches, and the correct one depends on your goạl. For example:
as KMT suggestions, if your goạl is to store data locally, Core Data is a good option
OTOH, if your goạl it to store stuff in a cloud account that the user already has set up, CloudKit is a good option
if you want to talk to a database managed by some enterprise, it’s generally best to front that database with a web service and then talk to that web service from your app
Share and Enjoy
Quinn “The Eskimo!”
Apple Developer Relations, Developer Technical Support, Core OS/Hardware
let myEmail = "eskimo" + "1" + "@apple.com"