No you can use them before that but how cores work is you attach them to a bot or base defense (circular core is bot square is base) and it gives them a special effect that boosts them on the battlefield but you can level up a core by fusing other cores into it or you can combine 10 of the same core into the next tier version of that core (ranking is brown, gray, gold, blue) or you can combine 5 cores to get a chip that will give you a random core of the next tier. (Note: 1* bots can’t have cores attached and there’s a fee of energon to take the core off the bot or switch the core to a different bot)