Apache2 issue with Website setup Ubuntu 19.10

im trying to set up a Website on Ubuntu Server 19.10 with Apache2, it somewhat works but not as I want it to. Currently I need to manually go into my folder and select the main.html that I want as first page to appear if you load into my website.

Im have no idea what the problem is and I spend a whole day trying to figure it out but I just cant get it wo work

My folder looks like this

/var/www/test.de/
                 HTML/
                     main.html
                     page2.html
                     page3.html
                 CSS/
                     css1.css
                     css2.css
                     css3.css
                 JS/
                    js1.js
                    js2.js
                    js3.js
                 resources
                         /images

Now if I try to set my RootDirectory to /var/www/test.de/HTML it will load everything but not the css/js/resources or what ever I tried to include in the HTML's

If I set it to /var/www/test.de/ I need to manually navigate into HTML and selete the file

the main Apache2 config is untouched btw

in my Directory Index is set to the main.html, if I set it to HTML/main.html I get only the main.html to appear at all, I cant access other pages but the css and js works just fine

Honestly im starting to die inside because of this, I hope someone can help me out.