During CBT your platoon is made up of all the cadets in your class for your academic year company (I do believe they may be shuffling you out of CBT, but don't worry about that right now). You go and join your academic year companies, one of thirty-six, after CBT and the squad designations you had during Beast are null and void.
Here is the structure of the Corps:
Brigade x1 (entire Corps)
Regiment x4 (~1100): ex. 3rd Regiment
Battalion x3 per regiment, x12 total (~350): ex. 1st Battalion, 3rd Regiment
Company x3 per battalion, or x9 per regiment, x36 total (~115): ex. Alpha Company, 3rd Regiment ("A-3")
Platoon x4 per Company (~36): ex. 1st PLT, A Co., 3rd Regiment
Squad x4 per platoon (~8)
Your company is the group of people, from all four classes, that you live with. Much of your daily life will be dictated by your company's culture. If you are not on a D1 team, you will eat with them as well. You go to class primarily with a random assortment of cadets from your class year, non-company dependent.
You will compete with the cadets in your class year in your company for military grading and you have a class rank among your entire class.
You do not need to worry about any of this right now; I am just entertaining your question. It will make much more sense once you start to see the structure of the Corps in action.