They used Nicker poster colors which are basically gouache just with different amounts of pigment and gum arabic than normal gouache with a focus on opacity and ability to lift the paint rather than lightfastness or archival properties (though Nicker is better at both of these than most generic poster colors).
They most likely have a detailed pencil layout to work from.
Working with paper that can absorb a lot of water without warping is key. They often wet the paper before starting. They work first with large brushes to get the general gradients and temperatures of the painting down quickly or the sky if there is one.Then they work from big to small, usually semi-transparent to opaque for details. Also generally they work back to front if you think of the layers of depth (i.e. they paint the furthest thing away first and layer on top of it) The semi-transparent step is light to dark but they can easily come back in with opaque mixes to use light paints over dark.
>Light and Dark: Art of Hiromasa Ogurahttps://e-hentai.org/g/1164516/ac2839da83/>Ghibli Artist, Kazuo Oga Creative Painting Processhttps://www.youtube.com/watch?v=gtPAFPv76ugThis is a rare example of an experienced background painter working but unfortunately it is a vignette for a personal piece rather than a more practical example of a background.
More examples from other artists:
https://www.youtube.com/watch?v=lYxOjgGdZXwhttps://www.youtube.com/watch?v=lSOFHmrIpvMhttps://www.youtube.com/watch?v=ix-hng0cEAQhttps://www.youtube.com/watch?v=9m1YUzDa0Eohttps://www.youtube.com/watch?v=N0kstijmv8Y 
