That's a lot of generalizations about the US,
Quite a big generalisation about the UK too tbf.
noticed in the UK that karate and judo are very popular . This I saw on ytube videos
YouTube is not a reliable source of information.
It'll also be heavily influenced by what is available locally...
Near me, it'd be (from most popular to least, based on number of schools/locations):
TKD (if counting both ITF and kkw)
Boxing
BJJ
Karate (of some sort)
Kickboxing
Aikido
Taichi (of some sort)
MMA
There's a few different things too, like I'm pretty sure there's at least one Kung Fu type school - no idea on anything more specific about it.
Not aware of a judo school.
There's a KM place that is borderline 'local'.
At least, that's what I've found over the past year or so. I'm sure there's more, but I don't exactly search and they aren't heavily publicised (and none to my knowledge have their own dedicated facility, it's exclusively clubs/schools run from rented part time spaces).
That's not indicative of the UK as a whole by any means - just what's within about a 25-30 mile radius of me. That's about an hour of driving each way and that would be seriously pushing it for actually attending. Anything over a 20 minute commute to class and nobody would ever turn up.