The UK application process is very straightforward: there is one application (via UCAS) for up to 5 universities: one essay (academic), 1 LoR & 1 fee.
As you have seen, for most US students at most UK unis meeting the stated requirements are likely to get an offer.
As for the ‘why’ go to the UK, there are several reasons. Some go for the adventure. Some go b/c the overall cost of the degree can be lower. Some go b/c some combination of the continuing assessment process of US high schools and their journey from 14-17 has resulted in a transcript with high test scores but a GPA that makes admission to highly selective colleges in the US unlikely. I think most go b/c they have a subject that they love and they want to study just that (vs the US model of a rounded education).
And that last part is the thing to keep in mind for your youngest: he really, really needs to be sure of his subject before he goes. The courses are much more structured and not as flexible as the US. For example, Edi offers courses in Electrical Engineering, Computer Science and EE + CS (and a variety of other related combinations). Your son could apply to all 3 (which would be 3 of the 5 allowed). The entry requirements are very slightly different, and the year by year coursework diverges as well. There can be some room for movement between closely related courses in the first year, but that’s it. The year by year curriculum for every course (at every university) is available online.
The other thing to keep in mind is that the UK system assumes a much higher level of independence than the US, and UK students are accustomed to managing the exam based structure. Be sure that environment will suit your son- while most of the US students that I know who go to the UK land happily- but every year there are a few who get there and find that it’s not all swishy robes and ancient buildings 