Think of “before the big bang” like “South of the South Pole.” It just isn’t a thing, you’re at the furthest point and it doesn’t go further.
And I don’t think there is a true “end” to the universe, as we understand it currently there’s just an expansion forever and at some point all the individual particles rip apart and spread out and nothing could possibly survive in such a situation so it counts as an “end” for all intents and purposes for us, but time itself is infinite.
IIRC the math actually can check out for an always-existing universe (instead of a big bang) but it doesn’t really make sense because you still then have to explain the giant sudden expansion.
I feel like I could talk about this for years, but I got video games to play. The short answer is I don’t feel like I have to know what caused the matter to all be at the same place and then expand to be satisfied with an infinite universe of finite matter. I wish my brain could understand how time as we know it started with the big Bang, but I think I’m slightly too dumb for that.
