It may depend on the journal. I can't say much about the first question because the journals I have been involved with didn't make this distinction. I'd think "confirmation" means a positive reply from enough reviewers that they'd do it, but I'm not sure. Note that it may only change after all reviewers have accepted, so it may mean they're still looking for a second or third reviewer after one or more having accepted already. It may even be that ultimately the editor decides to go with the reviewers they have, so that this status may change if the editor decides to not use a further reviewer (even having requested one earlier on), rather than another reviewer agreeing (as handling editor I usually invite one reviewer more than I really need, so that I have one to fall back on if one drops out; this means that occasionally I can finish the process before the last reviewer has even accepted).
Generally I wouldn't read too much into these status statements; under some circumstances they may have a quiet erratic relation to what really happens (in case editors use the system in a somewhat unorthodox way, which can happen).
"Does a reviewer start the review just after they accept to review it, or they must wait until enough reviewers to accept the review and then the process begins?" - A reviewer does not have to wait until there are enough reviewers, but this absolutely does not mean that they immediately start to review. People have other stuff to do, and reviewing is unpaid and doesn't count much on the CV, which means that reviewing has a rather low priority for most reviewers. Some do it in time, some start a day before the deadline, some need a number of reminders after the deadline, and some who have initially agreed even drop out, so that somebody new has to be found after quite some time into the process. The time it takes to collect all reviews has a huge variance.
