Can anyone help eli5 the free sprint deal w a new phone?

Hi, I'm trying to figure this out but am so confused.

I have an emergency verizon prepay flip phone, but I want to buy a smartphone now, and try the sprint free year deal. Obviously my crappy flip phone is not supported, I checked on the link I found here in this sub for the phones you can use https://www.sprint.com/en/shop/bring-your-own-phone.html?phone=byod (thank you guys for the link)

So anyways I can just go to a store and buy one of those phones? It would be new, and not connected to a phone number, but could I use the imei from the new phone I'm going to buy, and the phone number I currently have from verizon? Or would i have to activate the new phone to a service first?

I tried to research here and could only figure out that even tho the stores like best buy or wherever sell "unlocked" phones, if I connect to a service it would get locked or something, so I'm afraid of going out and spending $200 on a phone and then not being able to even use it with the sprint deal if I fuck it up since I don't know what I'm doing.

I'm not very tech savvy please I'd appreciate any help anyone can give me.