Is mastarbation is also beneficial like urine to flush out stones?

Not particularly. Stones will come out when they are ready. I know of no data that suggests semen would make it easier or have any effect on the process. If anything the increased viscosity might hamper stone movement.

There is no benefit or harm from masturbation. For healthy diet: http://www.choosemyplate.gov/., exercise 30 minutes/day, drink enough water daily so your urine is mostly colorless, have safe sex, no tobacco alcohol weed or street drugs. Get HPV and Covid vaccines. Wish you good health!