How can unlock an iPhone 8 so I can use it in another country?

Follow the steps here: link-> How to unlock your iPhone for use with a different carrier - Apple Support

If you are just visiting the other country, you can also contact your current carrier and inquire about a roaming plan for it in the country you are going to visit.

Contact the carrier it is locked to and request an unlock.

Not all carriers offer unlocking and the carriers are free to set requirements and in some cases, fees for unlocking.