may be the hierarchy is extracted and a number of layout cells are created and only the needed ones are then loaded? just curious.

Just to load a GDS file with "gds read", it will read everything into memory. But you can create "abstract views" of cells that point to sections of the GDS, and preload them, and select an option to tell magic to not read cells that are already preloaded. But it's not like a disk-based database format.

Thank you @Tim Edwards 👍