If I add a new product on my Amazon seller account, will it be created on QuickBooks Commerce?

No. Amazon does not notify QuickBooks Commerce if there are new products added to your seller central account.
QuickBooks Commerce pulls your Amazon product only after you set up your integration. After that, you should manually create the variant in QuickBooks Commerce with the matching Amazon SKUs.
This means that you will have to create the variant/product in Amazon and then create it on QuickBooks Commerce. That is two (2) times product creation, and SKUs have to match.