The idea behind a greenscreen is that you can make that green colour transparent in the frames of footage allowing you to blend that with some other background or other layered footage. This has issues like not always having a uniform colour, difficulty with things like hair, and lighting affecting some edges. These have to be manually cleaned up frame-by-frame, which takes a lot of time that is mostly busy work.

An alternative approach (such as that used by the sodium lighting on Mary Poppins) is that you create two images per frame -- the core image and a mask. The mask is a black and white image where the white pixels are the pixels to keep and the black pixels the ones to discard. Shades of gray indicate blended pixels.

For the mask approach you are filming a perfect alpha channel to apply to the footage that doesn't have the issues of greenscreen. The problem is that this requires specialist, licensed equipment and perfect filming conditions.

The new approach is to take advantage of image/video models to train a model that can produce the alpha channel mask for a given frame (and thus an entire recording) when just given greenscreen footage.

The use of CGI in the training data allows the input image and mask to be perfect without having to spend hundreds of hours creating that data. It's also easier to modify and create variations to test different cases such as reflective or soft edges.

Thus, you have the greenscreen input footage, the expected processed output and alpha channel mask. You can then apply traditional neural net training techniques on the data using the expected image/alpha channel as the target. For example, you can compute the difference on each of the alpha channel output neurons from the expected result, then apply backpropagation to compute the differences through the neural network, and then nudge the neuron weights in the computed gradient direction. Repeat that process across a distribution of the test images over multiple passes until the network no longer changes significantly between passes.