Example REST API back end

I was wondering if there are any good open source examples of a REST API back end. Mostly looking to see how things are done in a larger production environment. Specifically looking for apps that use a layered architecture, have validation (that requires database reads), a non 1-1 mapping between DTOs and database tables, endpoints that don't necessarily follow a basic CRUD pattern, etc, preferably in a Java/C# type language. The textbook examples of how to do things are way to basic for what I have to do at work, just looking for patterns/solutions to problems similar to what I have dealt with.