It depends on the seller. Sellers can opt-out of commingled inventory, but it incurs additional labeling/packaging costs for them. If they've done this, then you're getting the physical item they've sent in to FBA. If they haven't, then you may get the physical item they sent in, or you may get some other item that someone else sent in under the same identifier.

As far as I know, there is no way to know whether an FBA seller uses commingled inventory before purchase. The only option is to ask and hope they're telling the truth.

It's not 100% guaranteed, but if you receive a product and the barcode has been covered with a sticker with a code starting with X e.g. X123456879, then it's most likely from that specific seller.

Yep: https://www.amazon.com/gp/help/customer/display.html?nodeId=...