Hmm .. I do not have any specific video about this. This has been changing a lot between Altium versions, so it really depends on what version you have.
At AD20, you just click on a polygon, it should get selected. Hover cursor over edges and you can move them. If you need to add more corners, hover over edge, press CTRL and left click.
This may help a little bit:
https://youtu.be/L36KicrU45Q?t=1159Polygons are very annoying sometimes, I do not have any special technique for this, just, I try to draw them as the last objects on my PCB.
3) I am not sure what you mean ... like pouring for example GND into empty space on your PCB? I do not normally do that.