Question about buying an unlocked phone?

So I'm moving to Japan and don't want to purchase a phone over there (because they're usually more expensive). I do, however, need a Japanese phone number, which I can easily get by buying a SIM card. I was thinking of buying the iphone XR. How can I purchase an unlocked phone without paying for the actual phone service? Can I pay for the model in installments? I just want the phone itself.

IPhone XS Fats charger?

Can you use the normal lighting cable and simply switch out the block for a Samsung fast charger for fast charging in the iPhone Xs?

Is the iPhone XR good?

I currently, own the iPhone 8 and I'm quite satisfied with it, however, a while back I brought myself the iPhone XS and felt like it was too expensive and overrated so I'm selling it now. I'm quite happy with the iPhone 8 and glad that I've went back to it.

Why does instagram go black and crash my iphone?

No matter what I do. I've logged off and logged on. Deleted the app and installed it back. Restarted my phone. It just doesn't work. On top of that, iphone gets really hot after. Every other app works fine.