Short answer: we can’t.
Since we all share in each other’s limited perspective from inside this bottle we call “time and space”, nobody is going to be “explaining” this as if we could have some other studied perspective or experience. We only have our imaginations for what it might seem like to be “outside of this bottle”. But (speaking for myself), I want to abstain from the hubris that would be needed for me to presume that my/our experience is the only possible type of existence, and that nothing could possibly exist on the other side of any barrier from my/our understandings. For one thing, we can at least approach that wall from one side, even if we can never fully see or understand what may be on the other side. Some of us are willing to gamble that there is more out there beyond our reachable boundaries.
We can only use analogies (like ‘walls’) back to things within our own little bottle of existence to speculate how “being outside time and space” might make some sense – but we can’t really experience it; or at least I suppose not short of receiving divine revelations or visions. In one sense you are right to say “everything” must be in time and space … at least “everything” as it is commonly used by us, of course. But it is only another chapter of our old tribal hubrises to declare in a wider philosophical sense that the inconceivable is impossible. We’re like islanders who,since we don’t know anyone who has ever sailed away from the island we’re on and returned, conclude that no other lands could possibly exist.
Added edit … of course as a Christian, I think I have more than enough hints and intimations in Scriptures to warrant my belief that God is everywhere including beyond any of my limiting barriers of understanding. That doesn’t contribute to our understanding in any scientific sense, of course; it isn’t supposed to. But it is all I need for my resting point of faith. I’ll still use science or philosophy to know learn what I can.