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I want edit every single page in wordpress. Is possible?

Sorry if this question sounds stupid but im a noob with wordpress.
I am a javascript developer, and I want make a little game (or games) with wordpress. Opening main page there is a sliding puzzle. After complete, a button appears and clicking there you pass to another page. This is basically the same to all pages.
But I can't find were i put my javascript or HTML code. I create the pages but I can't edit them the way I want, everytime i try to do it i only can edit like word. I cant create, change or edit divs attibutes or tags. To be honest the only thing i can edit is CSS in stylesheet, but is too limited.
I have done the search, try found a solutions or ways to done it but when i look to tutorials is all diferent from what i see.
to give you an ideia I want to do a mix of this:
-> http://www.rustylake.com/room-escape-games/cube-escape-case-23.html
and this
-> https://ebonyriddle.com/
but i don't know how to edit page to page.
can you help me?
Thanks for everything
PS: I know I can do it without Wordpress, but I want use it anyway to learn more about the CMS (because of work) maintain website structure and later learn how to do my own templates or plugins.

To answer your question directly, yes it is possible (and easy, relatively speaking) to add custom code to every page / post of a Wordpress site.
More generally, you’re talking about creating a custom Wordpress theme, which you can find out more about here:
https://codex.wordpress.org/Theme_Development
and here:
https://developer.wordpress.org/themes/getting-started/ (Good suggestion from #ItsGeorge)
Regarding adding custom JavaScript to a theme, your files will need to be enqueued as per these instructions.
Outlining the steps involved in Wordpress theme development would go beyond the scope of your question, but there are plenty of resources online and answers to Wordpress specific questions on wordpress.stackexchange.com. You will need to familiarise yourself with php as well as JavaScript, to take full control of Wordpress.
I would say that developing a game feels like an odd fit... If learning Wordpress is your objective, a blog or personal website might be a better place to start, then you could move onto a game once you are familiar with the concepts involved, e.g. the loop, which plays a huge part in how Wordpress works
Good luck!

The primary focus of WordPress is to put HTML code and JS out of the user. You might manage your extra JavaScript files with your theme customization. If you want to get more control about how your pages are rendering, you should use action hooks and filters. The simplest way for you is to override each page template like this
https://developer.wordpress.org/themes/basics/template-hierarchy/#single-page
The preferable resource to ask Wordpress related questions is https://wordpress.stackexchange.com/

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For js you can use: https://marketplace.visualstudio.com/items?itemName=RobertHoffmann.FindUnusedFiles

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My question is how to protect my site from malicious code. I know Facebook has an option to put custom HTML in a page tab so it can be done safely. Currently, the html is being displayed by a php script that echos it onto a page so users can enter javascript in <script> tags as well. I don't know the full limits of javascript and html but I know that custom javascript embedded into the website has the potential to screw things up.
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