Are all iPhone 16 models unlocked?

No. All iPhone 16's are not unlocked. It depends on from whom the device is purchased. If bought directly from Apple and not tied to carrier subsidized plan and paid for in full or with an Apple Card on a monthly plan, the phone will be unlocked. If bought anywhere else, from say a carrier or a big chain retailer like BestBuy or Walmart, the phone will be carrier locked. If you want a fully unlocked iPhone, buy it from Apple directly. It does appear Straight Talk does support iPhone.

straycat23 wrote:

Are all iPhone 16's unlocked or can I use my Straight Talk Sim on any iPhone 16? Thanks

And additionally, if you’re buying in the US or specific other countries, you’ll need an eSIM and not a SIM.

No.