Your project idea sounds overwhelming for a first project. I recommend starting with a simpler project before attempting this project. If you really want to do this project right now, start with one smaller part, such as editing HTML, and moving on to things like WYSIWYG editing, video, audio, and sharing documents with users later.
Some technologies you need to learn to make this app include the following:
SwiftUI for the user interface because you want the app to run on iOS and Mac.
PDFKit for showing PDF files.
TextKit, and the NSTextView and UITextView classes. SwiftUI's text editor is very limited and does not support WYSIWYG editing. You would have to subclass NSTextView and UITextView to do WYSIWYG editing.
You might need to study AVFoundation for the video and audio files.
You might find the following list of frameworks for text editing helpful:
https://github.com/mattmassicotte/TextEditingReference