Fill bucket doesn't register lines from other layers

I was watching a tutorial from 6 years ago on shading and the guy was able to draw lines on a separate layer, then fill the selected area with the colours of shading. When I tried it the fill bucket wouldn't recognize the lines that weren't on the layer, making shading a pain in the ass. Is there any setting that allows the fill bucket to recognize lines from other layers?. the link to the video is this: https://www.youtube.com/watch?v=rph6rD-aXk0