@OsoPolar
Case 1: You're clicking Inspect on window.html - this opens the UI inspector on the left. I don't know what OS you're on or why you have your windows tiled like that, but it opens.
It opens in the "Sources" tab, which probably is the last one you used.
The guide (and my video) clearly shows you're to use the Element Inspector:
e1667816-c755-49c5-a7e7-b1ad7045b303-image.png
The page elements are on the Elements tab. Devtools will open Elements when using the Element Inspector and choosing an element to inspect.
This is basic Devtools inspector stuff, so if your level is at that point, you need to learn to actually use the Devtools first before there's any point going further.
https://developer.chrome.com/docs/devtools
https://nira.com/chrome-developer-tools/
Case 2: You open Quick Commands and try to type "Toggle Developer Tools" while on the vivaldi://inspect page.
This is NOT what I showed - I opened a new tab with Start Page.
And the guide says to use any internal page. In my video I open a new tab, as it works also from the Start Page when using Quick Commands.
Please pay some attention before saying "doesn't work".