when I buy an iPhone from Apple is it unlocked and can be used with GSM OR CDMA carriers?

Well, that depends on several factors. For example, your country of purchase (which is hopefully the same as the country it will be used), and the specific iPhone model.

All current models of iPhone will work on any carrier as they have dual cellular modems (look in the online tech specs under cellular and wireless). And in todays world of 4G LTE and 5G the old designations of GSM versus CDMA really no longer apply in any practical sense.

Whether the iPhone is sold unlocked will vary by where in the world you buy it. In the USA if you buy direct from Apple and pay for the device in full, it will be carrier unlocked (even if you have it preconfigured for your current carrier).