I have a Mk I Chromecast that I setup using the original Chromecast application on a win10 laptop. That application still seems to work but is now unavailable on the web. What you're apparently supposed to do now is to go to this URL using Chrome.
https://www.google.com/chromecast/setup/ and then choose "set up your chromecast using this computer". However, it never finds the Chromecast. Despite the fact that I can successfully cast tabs or desktop with no problems.

If I use my android phone, it can see the chromecast device and all its details. I'm just puzzled why it never works from the laptop Chrome.

In the forums there seem to be a few other people with the same problem. But none with exactly the same setup that I can see. In any case, the help from the Google support staff is at the level of "restart Chromecast, update chrome, turn on wifi, etc, etc".

WTF?