That sounds fascinating! Pulleys are indeed amazing tools that help us lift heavy objects with less effort. I don't have access to videos or websites, but I can explain why a single movable pulley allows us to lift twice as much.
With a single movable pulley, the load is attached to one end of the rope, while the other end is anchored. When you pull down on the free end of the rope, the pulley moves up, lifting the load. The advantage of a movable pulley is that it changes the direction of the force you apply. Instead of pulling straight up, you can pull down, which reduces the effort required.
The key to lifting twice as much weight is that the movable pulley also doubles the length of the rope you need to pull. This means that for every inch you pull down, the load only moves up by half an inch. Since you're pulling down with twice the distance, you only need to exert half the force to lift the load. This mechanical advantage allows you to lift heavier objects with less effort.
I hope this explanation helps! Let me know if you have any more questions.