Where do I learn about CodeIgniter core (I mean the 'system' folder)?

The closest thing is the application flow chart in the user guide.There was talk, a few years back, of building a more formal writeup on the CodeIgniter internals. That effort fizzled, from lack of available manpower. The driving file is system/core/CodeIgniter.php.

CodeIgniter 4, for all of its relatve richness, has a similar problem.  The core service mechanism is described in the user guide, and Lonnie Ezell has written a number of blog posts which would be relevant, but there is no architecture document. The driving file is system/CodeIgniter.php.