Kindergarten and preschool can look very similar from a distance, but they're actually quite different.

Preschool is basically early childhood education for kids aged between 3-5 years old, though there are some programs that accept as young as 2. You usually sing songs, play with toys, take naps, and learn very simple concepts like letters and numbers. It isn't optional, but it can cost a lot of money. Kids often go to preschool to better prepare them for kindergarten.

Kindergarten, on the other hand, is a school grade for 5-6 year olds, though a late cutoff can sometimes mean kids as young as 4 1/2. Yes, it is considered part of elementary school in the United States, even if it's the rare separate kindergarten school. The activities are rather similar to preschool; you still play with toys, sing songs, do arts-and-crafts etc. The curriculum, though, is a bit different; you start learning how to add and subtract, reading very simple words, 2D/3D shapes and their names, and how to write very simple sentences ("the dog ran away"). It used to be half day (morning and afternoon classes), but that's slowly becoming overshadowed by full day kindergarten. Hope this helps 😊👍