How can I keep Avada from changing HTML and CSS I put on pages. When I save a page, the system deletes my coding or change it to odd combinations of HTML and CSS.
You can write custom CSS code in Appearance > Theme Editor.
For adding css into your theme you have to write css in your theme css files. For adding css you can use theme css files using ftp or from dashboard you can add it through Appearance => Theme Editor (but highly recommend avoid this way to keep your site safe).
About page content add: if you want to add content in html coding then you have to use Text Editor (Text Tab instead HTML Tab in case of Classic editor). If you use Block editor then you have to select Add Block then HTML (Custom HTML) and put your html code there.
Best thing will be please contact your purchased theme Author through Item Support page and let them know. your theme Author will assist you.