It's hard to answer something like this. Even for Europe, it's hard to answer something like this. There's a big difference between Inverness in Scotland (47,000 people in the city and 63,000 in the urban area) and London. Are you measuring from the city center or from where things drop off to farmland?
In Europe, cities often drop off into farmland very abruptly. You have built-up areas and then very abruptly farmland. In the US, you can have suburbs where there's 1 house on an acre (4,000 square meters) going for a long time. Even 1 house every 2-4 acres (8,000-12,000 square meters) isn't unusual.
If you look at a satellite map between cities in the Netherlands, you can see that there isn't any sprawling houses between Rotterdam and Utrecht. Places like Oudewater exists, but it's dense. It might not be as dense as larger cities, but people don't have enormous plots of land. You can easily walk to neighbors and shops. In the US, a lot of people live in areas where it can be 5 miles to get to shops.
In the US, people often have the issue that the wired internet doesn't go up their street which might be a couple miles with only a handful of houses. In the US, they might face costs getting it from the street to their house because their house is set back a hundred meters from the road. This isn't super typical, but it's typical enough that it effects a lot of people.
I think part of the issue is that there is no real demarcation between the urban area and outside it in the US. I can look at a satellite view of a city in Europe and clearly discern city vs farmland. In the US, a lot of rich people left the city for suburbs where they could set land use policies that kept lower income people (and often minorities they didn't want around) out of their town (and out of their schools and out of their tax base). They still wanted to be able to commute into the city, but often passed laws mandating minimum lot sizes, minimum setbacks from the street for houses, and emphasized local control to prevent housing they deemed undesirable from being made. These places have fine wired internet.
But they kinda slowly melt into progressively more rural places rather than having a clear demarcation. The US's emphasis on cars and a certain cultural idealism around rural life means that there are people that want to live semi-rural while not really having a rural lifestyle or job. They just drive farther and cheap fuel and subsidies make that possible.
The lack of coherent land use policies means that the area where people consider themselves as part of an urban area can keep expanding. Maybe a developer wants to build new housing. The city and suburbs don't want more housing and so the developer picks a piece of land just past the last suburb. It's another 15 minutes from the city by car, but with skyrocketing housing prices, there are probably going to be some people who think it's a reasonable trade-off. What's the difference between an hour commute and an hour and 15 minute commute? Why shouldn't I trade 10 hours of my life every month for a larger house?
A lot of European countries have greenbelts around their cities where you aren't allowed to do a lot of development. It prevents things like the US suburbs which initially were thought of as 5-10 miles outside the city and now often go all the way to the next city creating a giant area that people commute from.
But these suburbs aren't the problem. The thing is that the suburbs then meld into really rural areas - but those really rural areas aren't that far from the suburbs that rich people live in and commute to the city from and those suburbs have big-box stores like Walmart and Target and they have high speed internet. If you live in a rural area that's 5 miles from such a suburb, you think "I'm not in the middle of nowhere! I'm just 5 miles from this place that has a Walmart and high speed internet and a supermarket!" If you're 5 miles outside a city in Europe, you're in farmland - but very often the houses all seem to be on one main road even through that farmland rather than up winding roads that lead nowhere (other than a few people's houses).
In the US, because there's this gradual drop-off in density, there can be this "why not just a little more" mentality. "Why not just a little farther out to get a tiny bit more land? It's working for the suburb next door that's just a tiny bit denser."
And a lot of Americans want to live in a rural area with acres of land, but in a commuting way. US policy often supports this choice - fuel prices, road subsidies, same-price delivery by the postal service, and billions spent trying to give them modern communications choices. That last one is key for this. The US is offering billions for rural internet and Starlink is going after that money.
Starlink is partly happening because the US government put a large amount of money available to anyone creating rural broadband options. The US Postal Service makes sure that rural areas can still get their deliveries.
I think "how far outside the urban area" is a hard question to answer because it's hard to answer where the urban area is in the US. The issue is that there's a lot of people that live in pretty rural areas in the US. Amazon and other deliveries are available. LTE and often even 5G is available, though often not with enough bandwidth at the moment to supply home internet (though companies are looking to change that). A lot of places with low density have fine wired internet. But when there isn't a clear demarcation of urban vs rural, there's still going to be a place where certain things stop, but people are going to wonder about the cut-off: I'm just outside the place that has broadband and it's only 15% less dense here.
I think that's the big thing. You might be 50 or 100 miles outside the city, but the gradual reduction in density while still being surrounded by non-farmers can make you feel like you're still a part of non-rural life while your neighbors and shops get farther and farther apart. Good wired home internet usually extends well away from cities in terms of miles (though often with annoying monopolies), but people really spread themselves out in the US.