Unlocked iPhone 8 plus

Yes, they are "unlocked" in the sense that they will accept sims other than that of the carrier to which they are branded, at least in the case of Best Buy.

The problem is the phones are subject to the US Resellers Flex Policy which means they will lock to the sim to which they are first activated with.

So for instance if you buy a Verizon phone and activate it with a Sprint MVNO sim it will lock to Sprint. You will find it very difficult if not impossible to change that.

Unless you intend to use the phone on the carrier to which it is branded you should not buy it from Best Buy.