When you put soda in the freezer, the carbonation level does not increase. In fact, the opposite happens: the carbonation decreases.
Carbonation is the process of dissolving carbon dioxide gas under pressure into a liquid, such as soda. When the soda is sealed in a bottle or can, the carbon dioxide remains dissolved in the liquid due to the pressure. However, when you expose the soda to colder temperatures, such as in a freezer, the solubility of gases, including carbon dioxide, decreases.
As the soda gets colder, the liquid's ability to hold carbon dioxide diminishes, and some of the dissolved gas begins to escape from the liquid. This is why you might notice bubbles or fizz when you open a chilled soda—those bubbles are carbon dioxide escaping from the liquid.
The perception that a chilled soda has more carbonation is due to the fact that colder liquids can hold less gas, so the release of carbon dioxide bubbles becomes more noticeable. The carbonation that was already in the soda remains the same, but it's more apparent when the soda is colder because the bubbles escape more easily.
It's worth noting that extreme temperature changes, such as freezing and thawing, can potentially cause a soda can or bottle to burst due to the expansion of liquids when they freeze. So, it's generally not recommended to freeze carbonated beverages.