Extending a framework (non-core) library

In the docs I see:

Creating Core System Classes - https://bcit-ci.github.io/CodeIgniter4/g...asses.html

and it lists the core classes that one has the ability to extend or replace, and how to do that. But after skimming through the docs, I don't see where it is mentioned the proper way to extend framework libraries. In CI3 we had the MY_ prefixed method, but I understand we will be doing this a different way, just don't see a suggestion on how and where to do it.

So, for example, how would this be done for the Session library ??

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