The HTML of a WordPress page is a mix of many sources. Parts of it come from plugins, other parts from the main template, others from the database, others from the core. They can also come from widgets, template options, and so on.
Unfortunately, WordPress pages are not on specific files that you can open and edit their HTML.
In most cases, the pages are Frankensteins’ monsters built from many different sources programmatically. And that’s not a bad thing – the power of WordPress comes from its extensibility, which is a consequence of this design.
To change an HTML on WordPress, you need to track what is generating that HTML. Then, you modify it – either via the admin dashboard or programmatically (not by writing a bunch of HTML, but by calling PHP functions and methods).
If you are lucky, you may be able to change what you want on the admin dashboard with no code.
If the content you want to change is not changeable through the admin dashboard, you will need to do some coding. As you pointed, it would be best if you use child themes to avoid updates to overwrite your changes.
But if you got to this point, you need to understand how to code for WordPress first. You need to understand hooks, actions, filters, and so on.
A good recommendation is the book “Professional WordPress Plugin Development” by Ozh Richard. He is famous in the community and even referred to on the official WordPress documentation. But be aware that the learning curve can take some time.