Is Magento Commerce provide a marketplace(multi-vendor) by default with Magento B2B? if not what is the best way to implement Magento 2 B2B and marketplace in Magento Commerce?
Magento is not build to be a marketplace out of the box. Not even the B2B suite (which is only a set of modules over the core code).
There are some questionable modules out there that can turn your magento instance into a marketplace. I only used one of them which should remain nameless, because I strongly advice against it. the code is far from being the best and it is full of bugs.
As a personal opinion, I strongly recommend NOT using Magento as a full marketplace solution. It is not built for that and the cost of making it behave like one may not be justified.
I suggest either looking for a platform that is intended to be a marketplace, or use Magento only as a sales channel and handle the marketplace features in a separate application based on the needs you have.