If I get an offer from JobBank in Express Entry profile and because of that job I get invitation, do I have to join that same job offer upon migrating or can I still get a better offer and work somewhere else?
There is no "express entry" visa. Express Entry is a process to select immigrants (permanent residents) to Canada under certain programs.
If you application for permanent residence is approved after you are invited to apply under Express Entry, you will receive a confirmation of your permanent residence and if applicable, a permanent resident visa. Upon an interview at a border crossing, you become a permanent resident of Canada. If you are already legally present in Canada and filed an inland application, you may also receive an electronic confirmation and become a permanent resident without further validation.
A LMIA job offer may add points to your Express Entry profile and lead to an invitation to apply. You are not obligated to accept such job offer, although you may validly promise to do so under certain circumstances (with civil consequences). The points are given because a LMIA demonstrate the need for your ability in Canada, not because you intend to work on this job. Under Express Entry, if your total points rely on the job offer, the job offer must be still valid when you submit your application for permanent residence (after you receive the invitation).
Once you become a permanent resident, you are free to undertake any (legal) employment. If you are already employed, you may leave such employment without affecting your status in Canada. You may still be subject to civil consequences if you violates a contract rule on e.g. notice period, but you will not suffer any immigration consequences once you become a PR.
The uncertainty and your freedom of choice as a permanent resident partly contribute to the fact that most companies are reluctant to give you a LMIA-validated job offer to someone outside Canada (but of course if they really need you some companies will still do it).