How were duties assigned to the temple musicians in 1 Chronicles 25?

Answer: By lot, young and old alike

The 288 trained musicians cast lots for their duties, young and old alike, teacher as well as pupil.

“Together with their relatives, who were all trained and skillful in the songs of the LORD, they numbered 288. They cast lots for their duties, young and old alike, teacher as well as pupil.”— 1 Chronicles 25:7-8 (BSB)