If an arbitrary remote user could edit the text of any website—without the permission of the maintainers of that website—there wouldn't really be a web to view. Not that most of us would want to visit and read.
You can edit the source code files for a website that you control, and can also create and edit a website built entirely from locally-stored files.
Viewing the HTML source for a website is feasible with Safari on Mac and various other tools. Probably the one that you have acquired too, though I'm not familiar with it.
Websites dependent on CSS and CGI and on dynamically-generated websites (Python Tornado, Apache Tomcat, etc) are somewhat more difficult to view without local access, or impossible to view.
Some of various introductions (and these at various levels) available:
https://developer.mozilla.org/en-US/docs/Learn/Getting_started_with_the_web
https://pythonbasics.org/webserver/
https://www.python-me.org/tornado
Etc.
ps: Editing hidden websites? Probably best to learn HTML and some tools first, and then some more about security and privacy, and then about Tor and related tools, and then about Tor hidden services.