Not sure why there is so much confusion here (or maybe it's just me)...
There are 2 iPhone X models: A1865 and A1901. A1865 has a Qualcomm modem. A1901 has an Intel modem. A recent test shows the Qualcomm modem is somewhat faster and better than the Intel modem under certain conditions. A1865 also has the advantage of being backward compatible to BOTH CDMA (still used when signal is poor on Verizon and Sprint networks) AND GSM (still used when signal is poor on ATT and T-Mobile). A1901 is only backward compatible to GSM. Therefore, with 2 strikes against it and the ability to now buy the A1865 without having to activate it on Verizon or Sprint while purchasing (i.e., "SIM Free"), many consider the A1901 dogpoo and and not worth buying in any case.
Note on "locking" and SIMs (activation): My understanding is that "locking" is a bit of a red herring in your decision process here. It's really just a procedure networks use to prevent specific hardware from being swapped to another network without authorization. Getting a SIM while buying your iPhone (i.e., choosing a carrier or "activating" at purchase) doesn't necessarily mean you'll get locked, although it might (I'm looking at you ATT).
SIMs are the ID card of the cellular world. All devices need one to ID and connect to the specific network. When you get one you're "activating" your device on the network. You might get one when you buy your equipment (in which case you might get it for free) or you might get one later (in which case you might have to pay for it - I'm looking you T-Mobile).
Unless your equipment is locked (see above), you can pull out your SIM and put in one from another network, but BE AWARE that unlike the "superuser" of iPhones (the A1865) not all iPhones are backward compatible to all legacy networks. Thus, for example, you might be able to stick a Verizon SIM into an iPhone you bought "with" ATT, but it might only work with 4G/LTE so if your signal isn't great and Verizon tries to switch you to CDMA 3G or lower, which does happen, you probably won't have connectivity at all (and might have to deal with that whole "can't talk and text" issue) or worse.
PLEASE anyone out there correct me if I'm wrong. I'd like to be totally clear on all this too. Thanks.