The atoms in your body are indeed ancient, with many of them originating from the early universe or subsequent stellar nucleosynthesis processes. However, your body doesn't create new atoms in the same way that stars do.
During the Big Bang, the universe underwent a rapid expansion and cooling, leading to the formation of hydrogen and helium nuclei. These light elements were the primary constituents of the early universe. Over time, through stellar evolution and processes like nuclear fusion in stars, heavier elements were synthesized, including carbon, oxygen, nitrogen, and many others.
When stars go through their life cycles and eventually explode as supernovae, they release these synthesized elements into space. The expelled materials, enriched with heavier elements, mix with interstellar gas and dust. Subsequently, this enriched material can form new stars, planetary systems, and ultimately life.
The atoms that make up your body were formed through these stellar processes and subsequent galactic evolution. As you grow and develop, the additional atoms required come from the food you consume and the air you breathe.
For example, when you eat food, your body breaks it down into its constituent molecules, such as carbohydrates, proteins, and fats. Through digestion and metabolic processes, these molecules are broken down further, releasing energy and providing the necessary building blocks for your body to grow, repair, and maintain itself. The carbon, oxygen, nitrogen, and other elements from the food you consume become incorporated into your body's cells and tissues.
Similarly, when you breathe, you inhale oxygen from the atmosphere, which is then transported through your bloodstream to be used in cellular respiration. Oxygen is used by your body to release energy from food and fuel various physiological processes.
In summary, the "extra" atoms needed for your body to grow and maintain itself come from the food you eat and the air you breathe. Your body doesn't create new atoms but rather incorporates and utilizes the atoms and molecules obtained from external sources.