For each signature the game spawns a gameobject with a linerenderer component that adds points whenever the mouse is dragged to a certain delta distance. the points on the component are logged in local space, and the object is updated to follow changes in the page's position, rotation, and sorting layer.
On each signature there's an invisible rectangular region that tracks whether the mouse is hovering over it. With Unity UI you can do this out of the box with an Event Trigger component. In the code there is a check that only adds points to the signature if the rectangle is currently detecting a mouse hover.
Oke... I've figured most part now, one more question, how did you change from draging to drawing the signature? I'm acttualy put collider on my mouse hover, and collider to my sign place, so when they collide, i can draw a signature, but it seems this method is not a clean solution for this case, as i draw, mw paper also move with me
If you're doing the signature region using Unity UI, you can have it so that the signature hitbox blocks raycasts by adding a Canvas Group component to it. This will make it so that whenever Unity detects that you're hovering over the signature region, it blocks it from also detecting that you're hovering over the paper object behind it.