> Scheme: (always present, not always visible)
Schemes are required by the URI/URL's grammar. I'm not entirely sure what "always present, not always visible" even means … those would seem to be synonyms. Perhaps we could infer "not always visible" to mean that there's some implied default … but there's not, not for a URI/URL. (For a relative-ref, yes.)
> Path: (always present)
… but this blows our implied default theory above out of the water. Paths are not always present: "https://google.com" is a valid URL, with no path. (One could say it implies "/", but … see the above.)
> It is recommended to place IPv4 and IPv6 addresses into brackets:
It is definitely not "recommended" … it's malformed to put IPv4 addresses in brackets, and it's mandatory for IPv6 addresses.
Just read the RFC? https://datatracker.ietf.org/doc/html/rfc3986
> Host: (always present, not always visible)
I'm trying to think of how you could have a URL without a host "visible". The network location is like the defining feature of a URL, vs. a URI.
But if a URI lacks a host, it's not just "not visible", again, there's no implied default there. "Host" might not even make sense, depending on the scheme.