I am wondering about replacing Java's 'extends' keyword somehow for dynamically extending a class based on a parameter(file, environment variable, db...basically anything). Is this even possible because playing with class loaders or calling constructors does not achieve this. I am not asking "should I use interface or superclass hierarchy" rather what is extending really mean under the hood in JAVA because there aren't any good description about it just the good old inheritance jargon:
https://docs.oracle.com/javase/tutorial/java/IandI/subclasses.html
The only way to "replace the extends keyword" is to dynamically create classes at runtime, which is entirely possible but non-trivial. Vert.x is a good example of a project that makes extensive use of dynamically-generated classes.
Java wasn't designed as a dynamic language in that sense. There are several dynamic languages out there (some of which can run on the JVM), such as JavaScript.
rather what is extending really mean under the hood...
Without getting into a long treatise on OOP, when you say Derived extends Base, it means that Derived inherits both the public and protected API of Base (which it can then add to) and also the implementation of that API. It means that code expecting to see a Base instance can accept a Derived instance, because Derived "is a" Base. This link is created a compile-time. At runtime, instantiating an instance of Derived involves all of the plumbing that instantiating a Base instance involves, plus then the added plumbing for Derived.