What happens to seed leaves when the new plant grows from the seed?

Seeds don't have leaves. Or rather, they don't have leaves that are visible to the naked eye. What happens to leaves does depend on what kind of plant is being germinated from a seed. It would help to know here what kind of plant it is that you are asking about in order to answer your question the best. Thank you!

For vegetables, the first leaves of the plant are curled up inside the seed. The seed sends out a root to anchor the plant. As the plant grows toward the surface folded in half, the leaves uncurl out of the seed casing as the ground helps strip the seed casing off the plant. Even if the seed casing does not come off, the plant leaves will uncurl to strip the casing once the surface is reached.

The seed will mixed into the soil after the plant was growing.

The seed will mixed into the soil after the plant was grown.