Unlocked iPhones for other networks

If you are in the United States, and buy an iPhone directly from Apple themselves,

• Phones that you finance using carrier financing (AT&T, Boost Mobile, T-Mobile, or Verizon) will probably be carrier locked.
• Phones that you finance using Apple Card Monthly Installments or Apple iPhone payments will be unlocked. You'l be forced to connect to AT&T, Boost, T-Mobile, or Verizon to start – but presumably will have the option to switch carriers later.
• Phones that you pay for in full, up front, will be unlocked. With these, you don't need to select a carrier – you can ask Apple to ship the phone with the option to "Connect to any carrier later."

I'm basing this on the ordering pages on the Apple site, where phrases like "And your new iPhone will be unlocked" appear and disappear depending on how you choose to pay for the phone.

If you're buying from another retailer, watch out for phones that let you choose a carrier, but then immediately lock themselves to that first carrier. Or for ones that are carrier-locked to a particular carrier from the very beginning.