If you need the international version, the USA version may share some frequencies but I believe that some are different as well.

Unlocking the device will not change this.

A device that is locked to a carrier is subject to be used on that carrier for a prescribed amount of time (usually one year) before a carrier will unlock it, and then they may also charge a fee.

It is not worth it in ny opinion.

You will still be stuck with T-Mobile 'crapware' and there may be more differences than that.

I did look on XDA about this matter, and although I am no expert, it seems that the device must be unlocked, rooted, and then a global ROM installed.

 

You won't be able to SIM unlock the device yourself. You would need to get assistance from a professional 3rd party business that offers SIM unlocking. We can provide a recommendation for a trusted company if needed

 

I think you can unlock the T-Mobile version of Oneplus 8. The mobile operator may refuse you, nothing surprising. Most likely, the reason may be the current debt. However, there are other ways to do this.
I had the same case where I was not satisfied with my mobile operator. I wanted to change it and went to them for unlocking. In the end, I got rejected. After some searches for solutions to the problem, I found a company https://priorityunlock.com/2022/02/25/how-to-unlock-unpaid-att-phone/, which deals with unlocking unpaid phones. The guys solved my problem, and I could switch to another mobile operator without any problems.