There are these cheap RJ45 -> HDMI, HDMI -> RJ45 adapters on Aliexpress like this one -
https://www.aliexpress.com/item/1005001577157733.htmlLooks like they are just connecting the HDMI pairs to the twisted pairs without any smarts in them (I guess just impedance matching) - there's a teardown photo in one of the comments in the feedback.
Is there a standard for this type of thing or is it just kind of works but is not standardized?
I cannot find a standard for this, there's ethernet over HDMI, more complex adapters which actually convert it to ethernet packets, but nothing for these dumb passive connectors.
Attached the photos you are talking about.
I don't think there would ever be a standard for something like this, as its not HDMI compliant.
A proper HDMI over CAT6 adapter has chips to boost the signal.
Such passive adapters could operate over short distances at best, Even HDMI seems to need active cables for any remarkable length (30ft+)
Any signal integrity problems would cause HDMI to renegotiate its data rate, causing intermittent video issues. Longest (active) HDMI cable I've used was was 50ft active cable to a rear classroom camera as AUX input for a Conference system.