In theory, this is specified in section 3.1 of https://www.ietf.org/rfc/rfc3987.txt (including the "MAY" section beginning "Systems accepting IRIs MAY convert the ireg-name component of an IRI as follows"). That covers the distinction between the domain and the rest of the URL.
The use of the page encoding is part of the browser behavior, not any RFC. It has been retroactively specified in https://url.spec.whatwg.org/#url-parsing