Doubts when it comes to managing WordPress using PHP code

Hello guys.

1.- I need to optimize webs at theme level: I need to know how to disable functions that I am not using in my template to streamline web load time (I don’t want functions that are not used to be called), Is there a way to achieve this?

2-. I need to optimize websites in general: from this I know a lot but at the level of plugins, optimization of images, asynchronous loading, implement some cache system, minimize CSS and JS files, delete files that are not used following a little what marks you the Google PageSpeed or similar web (although sometimes, at least, the pageSpeed, it works a bit regular I think).

3.- Modify specific page or posts: I need to know how to modify the pages and posts I have in WP but at the code level (HTML, CSS and PHP), I understand that WP is calling functions like getTitle, getContent etc, okay, but is there any way to see the pages I have in code? I’ve gone through the theme files but there’s no way.

4.- In general, learn everything you need at the WP backend level to add functionality to the theme, etc.

Having said all this, if you can guide me in any way or recommend me some kind of course or some kind of learning path I would appreciate it very much.
At the moment I have bought two udemy courses for 10€ and 12€ with very good ratings, more than 24h on development of themes and plugins and well, I’m going to soak up info.

Thank you! Regards.