My real problem in wordpress theme customization?

I can enter content, add photos, define the main page etc. but I can't edit a page as i wish.. for example i want to remove the youtube player in the body of the website, and replace it with a content slider. or replacing the div before the footer with the div after the slider. or copying a sample feature from... show more I can enter content, add photos, define the main page etc. but I can't edit a page as i wish.. for example i want to remove the youtube player in the body of the website, and replace it with a content slider. or replacing the div before the footer with the div after the slider. or copying a sample feature from a web page i like and pasting it in my own page. you know? editing the theme as i need.

My first problem is idk where to find the files for doing so,

My second problem is actually how to do it. what code to copy, where to put etc.

Can you advice me what to do?
Is there any source/learning center that focus on teaching wordpress theme customization and exactly teaches what i need?
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