Newbie Airline Pilot Career & Seniority Question.

Dear all,

I'm interested in perhaps entering flight training to be a pilot.

Couple of quick questions.

Are all airlines based on the seniority system, even in Europe?

Are there any exceptions to this system?

Does this basically mean that its impossible to move around companies,
and
you are basically stuck with one for life?

If this is true, how come there are some pilots who are able to move
around
even later in their career? This must mean some airlines don't operate
on
that system, yes?

Thanks
A.D