Creating CPT for Galleries W/O Plugin

Hi everyone,

I’ve been using wordpress for the entirety of my website’s life, and I’m a very happy camper. However, I haven’t been studying PHP that long, I’m trying to wrap my head around trying to create a custom post type for image galleries, specifically being able to categorize them into different entities, each having their own separate galleries within that which are all individual posts.

For example, a site I follow has it set up exactly how I would want it, you can view that here. I managed to create a CPT that would allow me to have the slug I want and everything, I just can’t wrap my head around having individual galleries within each game/property? Is there something I’m missing?

Any input would be invaluable, thank you so much for your time.