God is self existing. The Bible does not say how God came to exist. The Bible only says that God existed for eternity.
"Your throne is established from of old;
You are from everlasting." (Psalm 93:2)
"Before the mountains were born
Or You gave birth to the earth and the world,
Even from everlasting to everlasting,
You are God." (Psalm 90:2)
If it can be explained how God came to exist, then that would imply that God had a beginning. But God doesn't have a beginning. He has always existed. So we can't explain how God came to exist.
There are some things that are simply beyond our ability to understand.
And there are some things God doesn't explain to us:
"The secret things belong to the Lord our God, but the things revealed belong to us and to our sons forever, that we may observe all the words of this law." (Deuteronomy 29:29)
"Oh, the depth of the riches both of the wisdom and knowledge of God! How unsearchable are His judgments and unfathomable His ways!" (Romans 11:33)
"He has made everything appropriate in its time. He has also set eternity in their heart, yet so that man will not find out the work which God has done from the beginning even to the end. I know that there is nothing better for them than to rejoice and to do good in one’s lifetime; moreover, that every man who eats and drinks sees good in all his labor—it is the gift of God. I know that everything God does will remain forever; there is nothing to add to it and there is nothing to take from it, for God has so worked that men should fear Him." (Ecclesiastes 3:11-14)