Did you purchase it directly from T-Mobile brand new?  Are you a T-Mobile USA customer?  There are SIM Unlock Policy requirements that your device and account must meet before the device can be SIM unlocked by T-Mobile.

https://www.t-mobile.com/responsibility/consumer-info/policies/sim-unlock-policy

I'm from Pakistan I buy OnePlus Nord N20 5G . But this mobile Sim network lock can you unlock this device 

See the above best answer. Are you a Tmobile USA customer since that's one of the requirements.

Good afternoon, I bought an iPhone 8+ from a friend, but it turned out to be blocked by the operator, I am not a T Mobile client, and I live in Ukraine, I don’t know anything about the old owner, the phone is not stolen, can I somehow unlock it? imei-359401081128149. Thanks in advance for your answer.

No.  The only person that can unlock it is the original owner by making the request to T-Mobile once they verify their eligibility.

are you able to give me contact information for the former owner?

Abaolutely not!  Not even a T-Mobile employee is allowed to give out the personal information of a current or past customer.

No