Do tmobile sim cards work on Verizon iPhone 6s?

Here's what I mean, I bought a Iphone 6s yesterday on craigslist but the girl who sold it to me's carrier was Verizon
I have tmobile and I don't have a nano sim card
Can I go inside tmobile and ask for a new sim card and if they give it to me will it work or do I have to unlock it?
I've done some researching and came across something that said all iphones are unlocked going from the 5 and up. Is that true?

Not all iPhones are unlocked. That depends on each carrier.
Verizon has most of their smartphones unlocked.

I was using a Verizon iPhone 5C on T-Mobile just by putting in my SIM card. No problems at all. It just took a few seconds to recognize the SIM and I got service.

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