In Apple's Core Animation documentation, it says there are two rendering paths involved. From what I know, CALayer caches bitmap data of a UIView content. There are two ways of providing content of a CALayer. One is implementing drawRect: or other CALayer drawing methods, the other is setting a bitmap to the property contents of CALayer.
Here I'm wondering, what happens behind the scene if neither of the above two things is done? I believe there is a private drawing path UIView uses in this situation. What is this private drawing path consists of? How does it work?
The crux of CALayer is that it's GPU-backed. In modern graphics and animation, you want to minimize the number of times your bitmap data crosses between the CPU and the GPU. These operations are costly.
CALayer always uses a private drawing path, whether you use setContents: or drawRect:. In fact, the underlying plumbing of CALayer handles both of these in essentially the same way. When you setContents: , CALayer takes the image you gave it, and uploads it to the GPU via OpenGL (probably nowadays Metal) calls. When you drawRect: the CALayer gives you a context into which you can draw, and then it does the same thing with the bitmap data.
If you don't set contents or implement drawRect, you can still do things like set the layer's background color, border, corner radius, etc. This is being rendered by CALayer's GPU-based private drawing path.