There's only one scripture that comes close to answering the soul question. In the case of Adam, God breathed a soul into him. My guess is that's the allegory referring to when we gasp that first breath upon entering the world.
But that was Adam and he was never conceived, he was created.
My personal opinion is when it attaches to the womb, it has the first and only opportunity to become a human being. A frozen embryo is static. Whether for a year or a thousand years it remains exactly the same. Growth can only occur when it attaches. I think that is God breathing life. But that is only my puny mind trying to make sense of the incredible miracle of life.