How do I buy a smartphone?

Sounds like a dumb question, but hear me out. I'm 17, and ever since I got a job at a fast food joint I've been wanting to get myself a new phone with my own money. I currently own a cheap $50 Samsung that my mom bought for me, and I'm part of a family plan (Metro PCS) where the four members of my family pay a total of $100 for 4gb of data each, per month.
My question is if I wanted to buy myself a more expensive phone ($200 - $300), how would I go about doing this? Does the phone HAVE to be one of the ones Metro PCs has on display? What if I wanted to buy a used iPhone from somewhere else? Can I do that? How? Any info will be appreciated

It has to be unlocked to the AT&T, Cricket, T-Mobile, and Metro PCS system of phones so if used most previously Verizon or Sprint phones will not work.
I haven't tried buying a used phone form Swappa or ebay. I have tried buying used from Amazon and paid the extra in case it didn't work and it didn't work online sadly the merchant was out of phones that worked with my phone company, Project Fi, when I returned it.

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