This isn’t exactly a tutorial but you might find it interesting.
Please note that my flat color drawings aren’t technically just flat color because I usually put a few soft shadows on them - as seen in the 5th step here. Whether I’ll include that step kind of depends on how well the pic works without it.
When it comes to fully shaded pictures, I’ll use hard+soft shadows on top of the ambient shades. I usually add a lighting layer + rimlight on top of it all. It’s a pretty meticulous job, hence the price difference. You can see a tutorial about that over here.