Free upgrades to first class are domestic only. For international, one either pays with cash, miles, or for higher status frequent flyers there are certificates. Certificates are extremely hard to use; miles are hard to use unless you travel low season. But there's very little low season anymore.

Here's how it works at United: https://thepointsguy.com/guide/ultim...ited-airlines/
Both my wife and I had decent status one year (Platinum) and got some of their PlusPoints. We tried to use the PlusPoints multiple times as Platinums but most of them eventually expired unused.
Tickets in F have been cheap enough over the last few years that many just buy discounted F tickets.