What did an adult Brully do primitively upon reaching maturity? They engaged in endless fights, obviously. However, if they felt the weapon arms they were naturally born with weren't optimal to their tastes, they would change them.
At maturity, a Brully could extract the Billakin forming their hearts and gather it as a Spirit Weapon. Since they had three hearts, they took time extracting little from each of them. After extraction, Billakin will begin to accumulate in the first heart, until it is back at full capacity.
In this time period, the remaining two hearts would share the burden of upkeeping the body's functions. Little by little, Billakin would be extracted from the three hearts, one after another, until a suitable amount had been gathered.
