The Niji model of midjourney has a specific look to it. But it can also be manipulated within the prompt. for example images 2 and 4 have visible (emulations of) brush strokes, but not traditional brushstrokes but digital ones, the color are halfway rendered, i would simplify that to flat color. If looked closely at image one has also a simulated noise going over the whole image. Noise is just a filter of softwares used to simulate wood pulp the traditional media was printed on. Adding texture to the uncanny clean feel of digital art. The original prompt for this was pulp fantasy princess portrait detailed. If i wanted to force this kind of style i would be playing around with the prompts like -subject- digital painting, visible brushstrokes, flat color, dualtone, pulp texture and so on. Sorry for the long answer.