L-Arginine is an amino acid — a building block of protein — that your body normally makes itself, and is also found in many protein-rich foods (meat, dairy, nuts, legumes, grains, etc.).

Often recommended with water and after meals (as per generic supplement advice). 
Because of its role in vascularity and performance, some people take it before workouts — but remember effects vary.

L-Arginine converts into nitric oxide (NO), a compound that relaxes blood vessels, improving oxygen and nutrient flow to muscles.

Indirectly yes — improved blood flow and nutrient delivery may support muscle growth when combined with proper workout and protein intake.

Research does not confirm significant testosterone increases. It may help indirectly by improving workout performance.