The ATP exam is easy. You can self study for it using one or two reference books, one by Gleim & the other by Jeppesen (I think). All questions in the exam question bank are in the public domain, as are the answer options.
The only thing that isn't available is the FAA's version of which is the correct option. That's where Gleim et al. make their $$$. These books contain the entire question bank + answer options + which is the correct one & why.
The flight test isn't too bad either. It has two parts: a ground 'grilling' & the flight itself.
I found the ground part the most difficult when I converted an Oz ATPL to the USA ATP-MEL. Mostly because I wasn't terribly familiar with the FAA's rules & regs and general USA operating practices. It's an easy trap to answer the examiner's questions using your foreign licence knowledge base/experience instead of the still tottery American ones!
Otherwise the flight test is just like an instrument rating test - the certificate includes IFR privileges. You'll do stalls/steep turns/unusual attitudes etc + 4 approaches - two precision, two non-precision. Engine failures too, of course.
The precision approach tolerance is no more than quarter scale deflection (even during asymmetrics I seem to recall. It was over 5 yrs ago I did it).
Everything that is expected of you is published in the FAA's 'Practical Test Standard' published for the ATP.
If you don't meet the experience requirements for an ATP then you'll have to go down the FAA CPL route:
CPL exam + flight test, IR exam + flight test and eventually ATP exam + flight test.
Each certificate ie ATP, Commercial, Private is specific to the class of a/c in which you did the test: Multi-engine land, multi-engine sea, s/e land, s/e sea, rotorcraft etc etc. To get another class on your 'licence' requires another flight test to the standard of whatever certificate level you would like privileges. You would then hold two certificates.
For example, in my case with an Oz ATPL valid for m/e & s/e + IR valid for both: I did my ATP test in a multi so am only authorised to exercise US ATP privileges in a m/e land a/c. I'm not authorised to fly any s/e aircraft or m/e floats/floating hull types. To fly a s/e type I would have to go for a flight test at PPL, CPL or ATP level and then would only be able to exercise the privileges of that particular certificate in a single.
BTW, all FAA flight tests include an oral section, unlike the UK where they seem to consider the theory exams to be adequate.