Didn't Jesus already exist in Heaven prior to being born by Mary?
Ths gets complicated.
... The word became flesh ... John chapter 1. There's a sequence there. Although, as I proposed, that isn't accurate from the eternal perspective, only from the finite perspective. But since the intended audience is finite, it makes sense to describe it in those terms.
Sometimes in the epistles, Jesus is referred to as Christ-Jesus. This indicates the progression. Jesus is a manifestation of "Christ", a manifestation of the Word. It's the Word which pre-existed. That's what is described in the Christian bible.
But, in truth, if a person believes the story, I'm not asking you to believe it, just trying to answer your questions *about it*, all of it pre-existed. The whole story and every event that transpired. Each and everything that was, is, will be, all of it "pre-existed".
So even that term "pre-existed" becomes meaningless from an eternal perspective.
I think the best way to consider it is: God had a plan, in its brain, for lack of a better word. This plan included creating finite imperfect beings. God knew that they would fail, and so, part of the plan included the NT story and all of those events which would occur. Because God is omnipotent, once those events were "planned" they "pre-existed", they were "destined" to occur, nothing could stop it.
Once the idea of "Jesus" was called into existence in the "God-head" it 'poofed' into existence. But because God is eternal, there is no before, current, or after. So, "Jesus" the word-made-flesh always and forever existed, and always and forever will exist, unless God chooses to wipe it.
Hopefully this helps. It's weird, I know, because, as humans, its requried to really-really stretch to consider it from an eternal perspective.