paint bucket fill, how to, beginner that used to use photoshop

[mintintegral18]

ok, cool. how do I use the flat fill tool? And can this be done on a new layer? Below is some experiences that can be considered background info but I've screen shot this..... I think I like new layers because it seems very well suited to revert back to original black and white drawing and I can make different colored versions and quickly export toggling on off layers.... unfortunately, I am unable to use the fill tool for a flat fill, even ON THE SAME layer. I've found a mistake in the characters thumb. Definitely I've gotten more into Xara with this post. There are a few lines that overlap, looking closed, but are not connected shapes or lines. I TABBed and ENTERED around the entire shapes, until it fill black, this is how I found the gap. the gap, I think I fixed it, and Added the shapes into one.

every time I click Fill tool, nothing happens. Every time I click and drag fill tool, a black arrow appears, but nothing happens.

I have rediscovered Windows Paint tool has a bucket fill tool. It allows me to fill closed shapes with color rapidly with one click. maybe I missed something, but if I understand How is the Xara fill tool like it?

I've tried this cool technique, joining to a shape that has a fill I want, but bonkers and haywires. the LINE turns the color but does not fill with color. And the line width seems to shrink to 1 pixel, down from 8. And another haywire experience, I noticed when fixing my thumb of the character, the rest of the line doubled in width and the line I free draw, stays the same. https://www.youtube.com/watch?v=foji27nyj9M