There is phalcon repository:
https://github.com/phalcon/cphalcon
Master repo is LAST released version and any pull requests are not accepted here.
2.0.x is finished and we no longer accept pull requests here. We focus on 2.1.x now, on finish up 2.1.0 version and releste it, it will be LTS version. I hope that we will finally use semantic versioning(i created issue on github), so 2.1.1 and next versions will be only bug fixes and small changes, like adding some new method for returning already done in existing code.
I guess when 2.1.0 will be released then there will be 2.2.x as well for new futures - some of them are already waiting currently but i guess that dev team decided to postpone them to 2.2.x.
If you want to implement some new feature then you need to learn zephir lang - https://zephir-lang.com, there is zephir forum - https://forum.zephir-lang.com. Each PR must contain tests(if it's something new), updated changelog and docs if needed(documentation is generated from them). If you know PHP well and other languages then it shouldn't be a problem, keep in mind that everything from php doesn't work yet in zephir. It's not 1:1, few things are missed.
Also there are two directories for tests, use this one - https://github.com/phalcon/cphalcon/tree/2.1.x/tests, this is old tests https://github.com/phalcon/cphalcon/tree/2.1.x/unit-tests and in future it will be removed after all tests will be rewritten/redone.
Also when you are doing additional commits to your fork remember about squashing all commits into one at end, just to keep history clean.
Also if you are implemnting new future you should write about it in documentation too - https://github.com/phalcon/docs, i mean only somelanguage/ref
, api is generated from zephir classes - that's why you need to add docs to them for some simple code examples.