It's not entirely clear to me what type of behavior you are looking for, but if you are expecting to be able to customize HTML, CSS, and JavaScript in Kentico Xperience or Kentico 12 MVC the way it works in Kentico Portal Engine, that's just not possible.
Using MVC requires either:
- Templates be coded ahead of time in Razor
.cshtml
files
- Templates be designed as composable pieces that can be built up as Page Builder components (like Sections, Widgets, Page Templates)
Content Managers then will create Pages in the Content Tree and add their content to those Pages. Those Pages either have Razor Controllers/Views associated with them to render their content (and related content) or use the Page Builder to combine components that display content.
If you want to display Attachments in a Page, you can follow the instructions in the documentation.
If you want Content Managers to be able to upload HTML, CSS, and JavaScript files to be displayed/rendered on the site, you'll need to come up with a custom solution - potentially using Media Libraries and a custom coded solution.