As they say in this part of the world "Same same - but different!" It's the same subjects (broadly speaking) but for the ATPL you go into MUCH more detail. So you've got to do 7 PPL exams and 14 ATPLs, 21 in total.
The PPL exams can be done at your local flight school / club. They're administered by the instructors there.
ATPL exams are done at test centres. There's not many of them (test centres) and they're only run once a month.
Just to muddy the waters further. There are CPL exams as well! If my memory serves me correctly; if you do the ATPL exams, you don't need to do the CPL ones. I suggest you research the difference between an ATPL and a basic CPL.