God does not always grant our prayers. The primary purpose of prayer is to deepen pur relationship with him. God has his reasons for saying “no.” God is also timeless, not part of our sequence of time. Even if he already knows all and has his will, he has already accounted for our prayers.
There are different approaches to God’s actions in the Old Testament. Some are more direct, others are that the OT writers understood all of their cultural history in a religious context (if Israel was successful, then it must be because God said it was so). Either way, it’s important to recognize the distinction between God and creature, and what morality is. Morality is, in a sense, choosing to act in accordance with one’s nature. For a human, that means choosing to act like a human should act. Note that God (the divine natuew) has no obligation to act like how a human acts. His goodness is not dependent on it.
God is a loving God. He has made covenants with men and has upheld them even when men haven’t. He sent us a redeemer and desires to help us become perfect through our own cooperation. God does will the good of other things, but he does not necessitate that things always live up to it.
God is not a God of the gaps.
Only one God made the universe. There are no other gods.
As for what God is, that is an extensive topic. God is not a bearded man in the sky. He is not the most powerful being in the universe. The divine nature has no parts, it does not occupy a point in space, it is unchanging and eternal. It is the source and foundation of all reality. All things that exist or could exist are (or would have to be) caused by God. All things are in God in a manner most comparable to knowledge. God is beyond our reality, in a sense more real than anything else (similar to how you are more real than the dog you are imagining, but that’s a limited analogy). God isn’t a being, but Being Itself, subsistent existence. He isn’t a “god” (as in, he’s not a _type_of thing (that is a human, that is a rock), he simply Is. But that probably all sounds like gobbledygook if you haven’t really studied that type of theology.
We can know God exists by seeing that reality exists and making conclusions based on our knowledge of reality. There are various arguments: arguments from change, causes, contingency, composition, universals, grades of the transcendentals, teleology, the principle of sufficient reason. There are other arguments as well, ones I don’t argue myself, but I wouldn’t completely discount them .