Sigil can only 'open/save' .epub files. It's an epub editor. That's why those others are greyed out. If you're looking to get an odt file into an epub, you'll want to look at one of the several available import plugins available for importing odt/docx.
'Adding' html files is definitely the way to get new files into an existing (or new) epub.
As to your View being "full of code". That's how it's supposed to be. Book View and inherent WYSIWYG editing is no longer a part of Sigil. Sigil itself allows you to edit the xhtml/xml of an epub and view how it looks with the Preview window. If you require wysiwyg editing capabilities, then you'll need to set up PageEdit as the external xhtml editor.
You really should read through the Sigil user guides. Many of these things are explained there:
https://sigil-ebook.com/sigil/guide/