By Jesus was perfectly God when He became a man. He decided to hide His obvious God-ness, and decided to come like a servant, and was born like a normal baby. When Jesus was born, it was God, but like a normal baby.
That is about as simple as you can explain the vast depth of the intricacies of “he emptied himself” in Philippians 2:7.
As difficult as it might be to explain without grossly getting it wrong, it is not that difficult to understand. God isn’t helpless, but Jesus, being God, was a helpless baby when he was born, for Mary had to wrap him warmly and place him in the manger (Luk 2:7). God is all-powerful, but Jesus grew to be strong (Luk 2:40). Jesus could call angels down to protect him, but quoted the Bible instead (Matt 4:6-7). God never sleeps, but Jesus had to sleep after a day of teaching (Mark 4:38). God is always in a perfect Trinity, but Jesus was forsaken by the Father (Mark 27:46). God is self-existing, but Jesus died (Mark 15:37). Many more examples can be given where Jesus did not grasp His right to express His Godness.
Why did Jesus not come in the fullness of Divine holiness, glory, and power? To serve. To be a servant of those whom He created. To love to those whom He would otherwise judge. To save those who had sinned against Him.
The man Jesus looked like a man, acted like a man, and was a real man, but he was also fully God, emptying Himself of those great God-like traits to serve you and me.