Yes, you can continue to work your business after PR. No, there is no need for you to have a job offer to go through Express Entry or to find a job once you are a PR.Ok
Ok, and if someone goes without a job offer on express entry, is it necessary to find/have a job there? I mean is a job conditional to stay in Canada or express entry means you'll get PR for sure even if you don't find a job, and even if you start/continue a business?
They key thing with Express Entry is having enough points to be selected out of the pool. You can do whatever you want job wise.