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Code for adding text to every HTML file on website

So I have a website, https://coolkids.gq. I want to be able to add a certain line of code to every single HTML file on the website using another HTML, JavaScript or PHP file, but I am not sure how to do this.
How would I be able to write a script to add a line of custom code to every HTML page on my website?
Edit: As pointed out by charlieftl, I should clarify that all files are saved in either the /htdocs folder or in a folder inside of that.

yes you can, just make some folder named include or whatever and create for example loginForm.html sth like
<div>
<form>
....
</form>
</div>
and in pages where you want to put it use
<?php require('./include/loginForm.html') ?>; in place where you need it
Of course it need to be PHP page.
Good thing is to have also included header and footer in external html and include it on every php page ;D
if you want some js script in every page you need to add it manualy:
<script src="js/main.js"></script>
but it can be included in footer.

Universal header in HTML

I want to create a universal header (and footer) using HTML/CSS/Javascript so that if I update it to one page, it automatically updates on the rest. I also want to be able to choose which link is showing up with the active class on it. Also, I am using bootstrap. Here are some criteria I need to meet:
No PHP
No Server side (must work on github pages)
Thank you in advance,
Albert

You can use jQuery to .load a HTML file:
HTML
<div id="header"></div>
jQuery
$('#header').load('static.html');
Alternatively, you could use iframe to load your static header. See this answer for more information regarding the available techniques.

Maybe try Jekyll, this is a tool to generate static pages, also this is how the githup page handles your own personal page. You just follow the file structure and set the layout properties in your files, then you can use the universal layout (including header, footer etc.) in your page.
Here is a link to Jekyll.Jekyll

Just adding js script to typo3

I have the following problem.
I have a typo3 page without any template I made by myself, but it gets in some way the style and the behavior of the other pages (I mean navigation, footer and so on). Now I have written some HTML inside the page by creating an HTML element.
In this HTML element, I included some js-code, which uses jQuery. The problem is, that the page loads the jquery at the footer and my scripts are loading before (in the HTML element). So my script does not recognize jQuery. How can I add my scripts at the whole end of the page? I know, that it has something to do with templates, but when I create a new template for the page, the whole content disappears.
Would be nice to get any help.
Cheers,
Andrej

It is usually good practice to read all your JS from a single file placed in the footer of the page. Add this to the setup section of your page template:
page.includeJSFooter.scripts = fileadmin/js/scripts.js
Then remove the JS from the HTML template and put into this file. This file could hold all your custom JS and possibly even all the libraries you use on the page (if you are not loading them from a CDN).
Bonus: the JS doesn't have to be re-loaded on every page view but can be read from cache.
For reference: https://docs.typo3.org/typo3cms/TyposcriptReference/Setup/Page/Index.html#includejsfooter-array

I hope by template you mean a template record where you store your TypoScript? Otherwise this answer is not what you are looking for. :)
You can just add an extension template on your page that only adds to the rest of the TypoScript but does not override anything. To do so, go to the template module, choose "info/modify" in the dropdown at the top and use this button
Explanation: an extension template has the checkboxes for clearing the constants and the setup not checked and will not mess with the rest of your site's TypoScript:

How to dynamically remove all unwanted CSS and JS from page

I have an amateur sports website to maintain and I am not an expert in web development. The pages I can put on the website can only be in the form of predefined templates, something similar to wordpress.
The main website loads it's own CSS files and JS files which we do not want as it interferes with our customisations. These files change as and when the main administrators/developers deploy new ones to the main/parent website.
I want to dynamically block loading of all CSS files and JS files that are not ours and instead use our own custom CSS and JS files.
Currently I am doing something like this below, but as you can see I will have to update the file names whenever they change to something else and I would not know when they change unless I notice the difference on the webpages.
<script type="text/javascript">
$('link[rel=stylesheet][href="/assets/application-b51e2731e0d53e4d422.css"]').remove();
$('link[rel=stylesheet][href="/assets/print-a622ffc90d1232b126e42c.css"]').remove();
jQuery('head script[src*="application-7fd426f9bca208.js"]').remove();
</script>
Is there a better way to block all unwanted css / js files dynamically without me having to hardcode it like I have done above?

Instead of worrying about tracking file names, why not just target anything that's not yours?
$('link').not('.myLink').remove();
And just make sure you drop that class on your link:
<link rel="stylesheet" href="path.css" class="myLink" />

Drop-down Redirect - Submit

i am using wordpress and i would like to create a drop down menu with select options and i found this thread which i think is exactly what i need Deactivate the 'Submit' button for the selected value in dropdown menu to be honest i don't know how to set up these elements html,js,css in order to achieve the result.I am a begginer and i would appreciate any kind of instructions.
Thank you for you time!

For instructions in HTML, CSS and JS go to the W3Schools website. I learned almost all that I know from there. If you need specific help THEN come asking here.

Let me assume that you are going to implement this in your WordPress themes index.php file. So, copy and paste the HTML code in to the html tag wherever you want to display the drop down box. Also copy and paste the javascript in to the script tag inside the head tag of the page. You can also paste the javascript code to a new file created inside the js folder inside the themes folder and call it from the head tag in index.php file. If you have any CSS you can either place it directly in the style tag inside head tag of index.php file or create a file in the css folder,copy the css code to it and call the CSS file from the head tag of index.php file.