Advice for US student coming here?

[Chilldonaldchill]

A friend has a child who would be starting uni in 2022. Both parents are from the UK but child was born in the States and has lived there all their life. They are considering both UK and US universities. They are very very academic; I don't understand the system there but they are apparently right at the top of rankings across the country.
We are helping them navigate some of the UK system such as UCAS, good websites for comparing courses and the online open days - but is there any specific advice for international applicants?
They are very likely to apply to either Oxford or Cambridge - is there data for international applicants so that we can advise on statistics (even though I know as individuals those are quite meaningless)?
If your child is applying/did apply from overseas is there anything you wish you'd known that would have helped them?
Thank you!

I posted a long reply earlier which has disappeared. Thanks everyone.
@seasonfinale I'm thinking it will be a bit if a shock for DC as well to suddenly (presumably) go from bring straight A student to more run of the mill - but thanks I'll make sure they know. They feel quite level headed i think. Do you think it's more of an issue for American students because they are used to quotas for top marks? I don't know how it works there.
@ellmau yes they have already done their APs and have straight 5s. They want to do history. I've not found any data on applications from overseas students by subject although I'm sure it must exist.
@DeathByWalkies thanks yes they do know. I don't know their financial situation but it doesn't seem to be an impossibility for them. I get the impression that - unless they go to an instate uni - the total cost of coming here for 3 years is not hugely different from attending an Ivy League there for at least 4 and probably more. There are also relatively well-off grandparents here who would be offer the job to have a grandchild here and who could provide accommodation for all holidays that they weren't going home. But I think they will apply to both and really not make any decisions (assuming they have decisions to make) until they really need to.
Thanks for all the advice though. It's really helpful.

@Themeparklover yes DD is discussing London with them. There are relatively fewer "top tier" unis for history though in London compared to sciences etc.

Dd has a number of friends from US at Edinburgh, taking a range of subjects. Bear in mind Scottish unis offer 4 year courses. Although even then it seems financially comparable to US.

We were shown round St Andrews by an American girl who had won a scholarship from the American Postal Workers Union (for children of postmen), according to her it covered fees and living expenses in Scotland but only fees in the US so she chose Scotland, it can definitely be cheaper even paying international fees.

DDs American flatmate in similar situation. Won generous scholarship but not enough for the US unis she was interested in. However enough to cover 4 years at Edinburgh with 2 return flights a year.

DDs American flatmate had stellar APs and had done really well in 1st year (70+ in humanities). Just like everyone else has to adjust to UK grading.

I’ve done recruitment events in the US. Our overseas fees are well below what most parents expect to be paying for an equivalent university in the US. Another difference is that the vast majority of U.K. halls of residence rooms are single, in the US sharing with up to three roommates is normal.