You can ask a dozen developers how they work with themes or plugins and you’ll get a dozen different answers.
My method is to use NetBeans to edit php files stored locally on my computer and then ftp (NetBeans upload) them to a test server with wp_debug turned on. NetBeans turns the theme or plugin files into projects and from there you can search all files in a project. I also use a maintenance mode plugin on the testing site so that if anyone visits the site they don’t see me monkeying around with code or errors. Yes, I could do this with a LAMP stack on a machine on my network but to me, that’s a pain to maintain and get set up. For $5 a month I can get hosting with any version of PHP and MYSQL with PHP MyAdmin to make database changes.
As for documentation and training, that depends on your knowledge of PHP. If you know PHP well enough then the only thing you need to worry about is WP specific syntax. If you see something in the code you don’t understand, use your Google IT department.
https://developer.wordpress.org/
That’s your bible. Every function, hook, class or method used in WordPress is there with the complete code and usually, some examples.
Good luck.