There is a search function at the top, also simply googling what you want to know will usually come up with Pprune threads.
In essence if he is already going for a PPL then that is the first step towards a modular licence. He will need to complete ATPL groundschool (Airline Transport Pilot Licence) and then complete his commercial pilot licence and an instrument rating.
The instrument rating is the most expensive bit, since for it to be valid on multi engine aircraft, it must be taken in one, overall cost for the whole lot is going to be around £60k.
Once you have ATPL ground school + CPL + multi engine IR you have what is generally referred to as a "frozen ATPL". This isn't a legal term, but is used industry wide to describe a CPL with IR and ATPL studies.
In theory you would then get a job, work for a couple of years to build 1500 hours and 500 multicrew, you could then do a test and apply for a full ATPL. Don't get confused, you will not have a full ATPL until you have decent airline experience.
The reality is very few airlines are hiring people with a frozen ATPL and the bare minimum of 2-300 hours. you would therefore need to look at other avenues such as instruction (£7k for the rating) or other smaller single crew jobs you could get to build more experience.
In terms of going to America, I have no idea on that but you would have an FAA rather than an EASA aircraft, which means European airlines wouldn't be interested until you did a conversion course.