What is the ruling on Salatul-Istikhara, and how is it determined thereafter which of the two matters the worshipper should choose, and does Salatul-Istikhara change anything of divine decree and destiny?

Istikharah is recommended. After performing Istikharah, the person should proceed with what they feel comfortable with, or even what they do not feel comfortable with, as all of it will contain good. This is because Istikharah is a supplication (Du'a), and Du'a is related to divine decree (Qadar), by which it can be altered.