Happy to explain. In my case the catheter is put through penis to the urinary bladder, I think. My catheter is small, but can be extended by another tube. The catheter is tied to the thigh. The leg bag is connected to the catheter. The bag will store the urine upto 800 ml. The valve of the leg bag is used to empty the bag. In my case the bag is supplied by the hospital. The flow of the leg bag is one way and no return.
Individual cases may be different. All the best.