How do I avoid the HTML I enter is altered? I use drupal 8.4.3 with ckeditor 4.7.2 (apparently)

When I press the "source" button, I want to be able to write whatever source I want, in just one line if I want to, I don't want the ckeditor to touch my source code AT ALL.

How is that possible? I think I have used 3 hours googling, without making it work completely. 

I want to be able to write "&lt;br&gt;&lt;br&gt;" without it turning by code into:

&lt;br /&gt; (-- LINE BREAK)
&lt;br /&gt;


I don't want it to touch it!!! it should just be &lt;br&gt;&lt;br&gt; (without the closing /) 

when I write into the source, ckeditor shouldn't touch it! How is that possible?
It's probably not just CKeditor altering your HTML. You should look into the configuration of your input formats at /admin/config/content/formats. There is documentation on https://www.drupal.org/docs/8/core/modules/filter/overview, although it's not very detailed.

An input format applies one or more text filters to your input. Common filters are the HTML Filter, that allows only a limited number of HTML tags, and the Line Break Converter, which turns line ends into &lt;br&gt; or &lt;p&gt; tags. You can configure your input format, or create a new one, without any filters.

If I recall correctly, even if you don't have any text filters configured, Drupal may still strip some HTML tags that are considered particularly dangerous, like the &lt;script&gt; tag.