Is reliance upon Allah Almighty a reason for something to happen, or not? And if not, how do we understand His saying: "And whoever relies upon Allah - then He is sufficient for him"?

Reliance on Allah is a means to achieve what is sought, but it is not necessary that it be achieved in the manner desired by the one who relies. Rather, Allah chooses what is best for him. Reliance is a state of the heart that arises from the servant's knowledge of his Lord and His uniqueness in creation, governance, harm, and benefit. Among the fruits of reliance is contentment with Allah's decree. So, reliance precedes it, and contentment follows it. And whoever relies on Allah and is content with His decree has fulfilled servitude. This is what the prayer of istikhara indicates, which combines reliance and delegation before the occurrence of the predestined, and contentment thereafter.