Hi, Gaby!
Thanks for posting in the Apple Support Communities. We understand you'd like to get some information on obtaining an iPhone that has been unlocked for use on another network. We're happy to help!
First, check out this FAQ and answer from iPhone - Buying iPhone - Apple:
Every iPhone you buy from us online or in an Apple Store is unlocked, with the exception of a few carrier financing options. With an unlocked iPhone, you’re free to go with the carrier of your choice."
If you happen to purchase a device that is locked, you can speak with the carrier about options to unlock it: How to unlock your iPhone for use with a different carrier - Apple Support.
Best!