Things I learned:

Miscelaneous

• When you define an inherited scene which inherites a scene extends the Area2D or KinematicBody2D, the parent scene must have a collision node, otherwise you can't call user-defined methods of that scene's script. In the inherite scene, define a shape for that collision node.
• The Timer must be added into scene to be able to work.
• Create an infinite background: See this.
• If the pointing direction of Sprite A is not the same as the positve direction of x-axis (the angle between them is angle in radiant), then when assign value to rotation attribute of A, you should consider to subtract angle to get the desired rotation value.
• You can set the rotation attribute of Node2D (or its child class) object A with value of rotation attribute of PathFollow2D object B, which belongs to a Path2D, to rotate A when Path2D changes its direction (notice the note about pointing direction of Sprite above).

Infinite background

This is an instruction on how to create a background scrolling downward infinitely.

• Create a scene with this hierachy: Node > ParallaxBackground > ParallaxLayer > Sprite.
• Import an background image (this image can be a small image, we can repeat it to fill the entire background). Godot will import it with Repeat and Mipmaps flags disabled. You should select that image, inspect it in Import tab, enable Repeat and Mipmaps flags, then click Reimport.
• In Sprite, load the background image.
• To repeat the image to fill the background, reveal the Inspector/Region panel, check the Enable flag, and fill the w and h field of the Rect with the width and height of the screen size of the game.
• In ParallaxLayer, reveal the Inspector/Motion panel, edit the Mirroring property. In my case, I want the background move downward, so I will set the y field to the height of the screen's height (I misunderstood that the value of x (or y) field should be double of the screen's width (or height) and set that wrong value, then a blank gap (which the height is the same as the screen, if I'm correct) appeared between the original background and the mirrored one, so don't do that :D).
• Create a script for the Node. In the _process() function, increase the scroll_offset of ParallaxBackground with a Vector2, which represent the scrolling speed of the background. My background moves downward, so the value of that Vector2 is (0, 2).

const BACKGROUND_SPEED = Vector2(0, 2)

func _process(delta):
  $ParallaxBackground.scroll_offset += BACKGROUND_SPEED

Play the scene, then the background should scroll infinitely.