The Trinity is difficult for anyone to put into words, and just for us to understand. We don’t fully, as God hasn’t fully revealed everything to us.
Do Catholics believe that through Mary the eternal God was created?
Or that Jesus didn’t exist until He was conceived by the Holy Spirit in Mary’s womb?
Neither. But your mention of the Trinity is illustrative.
I would recommend following Martin Luther's admonition:
[S]he became the Mother of God, in which work so many and such great good things are bestowed on her as pass man's understanding. For on this there follows all honor, all blessedness, and her unique place in the whole of mankind, among which she has no equal, namely, that she had a child by the Father in heaven, and such a Child.... Hence men have crowded all her glory into a single word, calling her the Mother of God.... None can say of her nor announce to her greater things, even though he had as many tongues as the earth possesses flowers and blades of grass: the sky, stars; and the sea, grains of sand. It needs to be pondered in the heart what it means to be the Mother of God." Luther's Works, 21:326, cf. 21:346.