Hi Ali, welcome!
To answer your question: every country or group of countries in the world has a different licensing authority. In the USA it's the FAA, in Europe it's EASA and in Qatar it's the QCAA. Generally speaking you aren't allowed to fly in an aircraft registered in a country, unless you have a licence issued by that country. I.e. if the plane has an N-registration you need an FAA licence. If the plane has an A7-registration you need a QCAA licence. Admittedly there are some exceptions, but normally you need to convert your licence if it is issued in a different state.
If you decide to complete your training in the USA to obtain an FAA licence, you'll need to convert it to a QCAA licence, before you can work for Qatar Airways.
Please be advised that obtaining a licence is the EASY part of becoming a pilot. Finding your first job is much, much more difficult. Get as much information as you can, before you start.
KA.