GDSII/GDS2 is a binary file format that represents the layers needed to produce an ASIC.
In the OpenLane flow, Magic is used to ‘stream’ the final GDS2 files.
All shapes are assigned to a given GDS2 layer, and each layer ends up being used to create a mask, although often this may involve the combination of one or more GDS2 layers to form one mask, and the shapes are often grown, or shrunk, or merged and thus what ends up on the mask may not be the same as what was drawn by the designer. Each mask is then used in a photolithographic step to produce the chip.
We can use other programs or even software libraries to generate valid GDS2 files. Here I’ve used gdspy to convert the Hackaday logo into GDS2.
You can check the very simple program out here
The Skywater PDK defines all the layers that we can use in the GDS2 files.
The course itself is very self sufficient, and on top of that you have the community behind it. People are happy to help and review your work. I would definitely recommend the course!