NNS supports both. If you go to https://nns.ic0.app or https://nns.internetcomputer.org it will use the same domain for sign-in accordingly.
Correct. Like I said above, it’s an optional parameter of the AuthClient
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Yes it’s possible, I set up my personal anchor few months ago and it works on both, but, I’m not sure it is well documented or even still documented at all. There was a tutorial shared in the incident thread but, cannot find the reference anymore.