I tend to believe God wrote the laws of physics and designed the eternity of our universe (including us). Once the design was in place, pushed the button and creation started with a bang.
I believe Genesis explained creation with simple symbolism that catered to the knowledge and wisdom of his people at the time (10K to 12K years ago). The story came before there was written history and had to be passed down many generations before a medium was developed that would capture it in writing. I believe it is true and the word of God but interpreted factually incorrectly. For instance, I don’t know how long one of God’s days is. How does eternal existence measure “time” before our earth rotated on an axis once every 24 hours?
But I do respect every believers own faith and opinion on the matter.
2 Peter 3:8
But, beloved, be not ignorant of this one thing, that one day is with the Lord as a thousand years, and a thousand years as one day.
FWIW, In Genesis, it plainly states "And there was evening, and there was morning, the 'Nth' day" 6 times. If the day was longer than 24 hours, say 1 year (or a billion years, as some silly people suggest), for instance, that means that plants on day 3, and birds on day 4, and land animals on day 6 had an awfully long night!
Exodus 20:11 plainly says he created the heavens and the earth, the sea, and all that is in them, and rested on the seventh day. This was to be the model for the week, and the reason for the Sabbath. Interpreting a day as anything longer than 24 hours, or one revolution of the earth, is an unnecessary compromise with science so called, and implies that Genesis is deceptive or even erroneous. We all get to choose what we believe, and we get held accountable for it.
Regarding what happened before the first day, nothing, for there was no time, not as we know it. That's why it's called the beginning, where God created time (beginning), space (heavens) and matter (earth) first, and then made everything else from there. After he made light, the night/day clock began, and has counted more than 2 million days since it started.