A:Answer Only if it is an "Universal Unlocked Phone". Below is the Best Buy link with the best explanation of unlocked phones and various carriers I've seen in a while.
https://www.bestbuy.com/site/mobile-cell-phones/why-buy-an-unlocked-phone/pcmcat284300050008.c?id=pcmcat284300050008
It seems most use GSM networks, or CDMA.
GSM carriers in the U.S. are: AT&T, T-Mobile
CDMA carriers in the U.S. are: Sprint, Verizon
So it's easier to take a phone from Sprint to Verizon than it is to take a phone from Sprint to AT&T, because you're not jumping network types. Also, swapping phones on GSM networks is easier because GSM-compatible phones use SIM cards.
In terms of their relevance to 3G and 4G LTE bands and frequencies, the thing to know is that CDMA and GSM only use 3G technology. So the CDMA vs. GSM discussion is really a 3G discussion. However, it's important to ensure that your phone has ample 3G frequency compatibility, since this is the network your phone will jump on in the absence of a 4G signal. Otherwise, you'll simply have no reception when you can't reach a 4G network.
But be aware it seems to be harder to get full coverage insurance that includes if your cell is lost or stolen if unlocked.