The function you are looking for in Sigil is called "Preview". It is a window that gives you instant visual of what you are typing in the code view window. It is 1000000001% better than the old page view that was very buggy and caused some bad code to be generated unless you were very knowledgeable about the subject. e.g. The current version is much better for newer users.
The only difference you need to be aware of from the directions on that video are instead of double-clicking on the page tab to switch between code view and page view, just hit the F10 key (or click on View/Preview in the menu) to bring up the preview window. Then you can dock it to the side, or keep it modal and place it anywhere, even on a 2nd monitor. When you close and reopen Sigil it will remain in its previous position.
PeterT is referencing an optional companion software to Sigil which is a basic WYSIWYG text editor. You can use PageEdit, or any other 3rd party html editor via Sigil's 'Open With' functionality, but it is not required. If you designed your whole book (Awesome!) then you certainly either already have, or can easily learn, the
basic html/css knowledge to work on Sigil's codeview page.... we're talking basic stuff like putting paragraph tags <p>...</p> around paragraphs to designate the beginning and end of a paragraph, or heading tags <h3>...</h3> to designate the beginning and end of your heading (like a chapter title). (see the attached image)
There are also some helpful guides/tutorials on
MR's Wiki page that may help...just be aware that some info may be slightly outdated.
Cheers!