All cells in the human body come from special cells called stem cells, which are unique in that they have the ability to:
1) become any specialized cell, such as a muscle cell, in the body and
2) to make stem cells just like it.

You can think of human development like a ball rolling down a hill, where the stem cell sits at the top, with the ability to roll in any direction. As time goes on, the ball becomes more committed to one path and is less able to, say, roll around to the other side of the hill. Cells are the same way, where the more specialized they get the less ability they have to become other types of cells. For example, a muscle cell can't just decide one day it's going to be a skin cell.

Instead, we rely on stem cells dividing to make new muscle cells, and other stem cells dividing to give us new skin cells. In fact, we can see the differences between skin and muscle stem cells as early as when an embryo looks like a big, hollow water balloon full of cells, called a blastula!

Muscle cell development is especially cool because of one of the most important things you need as a muscle cell: energy. So muscle cells are actually formed by fusing multiple cells together, resulting in multiple nuclei and lots of mitochondria in the muscle cell and helping it provide all the energy needed to move!