(1) Here’s a YouTube video about the problem
The scammer takes remote control and modifies the page while hiding that from the user, so the user isn’t personally in the DevTools.
(2) Some sites freeze when you open the DevTools
This is done with something that pauses execution in the debugger, but I haven’t looked into the specifics.
(3) At least one site seems to reject HTML changes
I have a userscript to modify search results pages and it has all kinds of inexplicable failures on Bing for reasons I haven’t tracked down. There may be some kind of halfway effective integrity checking in the page.