It is advisable to purchase a phone in the country where you intend to primarily utilize it. If you plan to travel, an unlocked phone is essential to enable seamless usage with a local SIM card across different regions. Notably, there is no international version of the iPhone, and consequently, there is no international warranty coverage. The warranty is exclusively valid within the country of purchase.
Having said the above, the answers to your questions are below:
Is there any quality variation between iphone Japan and USA or any TRA version?
You did not mention the iPhone model so I will assume that you meant regarding the iPhone 15. In the US, the iPhone 15 is sold without a SIM tray. The US models only accept eSIMs. iPhone 15 purchased outside of the US will have a SIM card tray and be capable of eSIM activation. There is no quality variation between the two models.
Or any issue with spare availability?
Is the question related to spare parts? If YES, then that will depend on where you take the iPhone for service.
Now i am in dubai so have any issue when i use the phone here?
You will not have issues as far as the iPhone functioning properly overall. NOTE that Dubai may have government regulations that are different from US and that may limit full functionality of the iPhone.
Axel F.