Four-color process works like a magazine page. Each of the four inks is printed as a halftone, and where dots overlap, the transparent inks combine: cyan and yellow read as green, magenta and yellow as red. The design does not need a screen for every color it contains, only one per process ink.
Because the inks are transparent, CMYK depends on a light background and looks best on white or very light garments. On darks it needs a white underbase, and even then simulated process usually gives brighter, more controllable results. Exact brand colors are hard to reach with CMYK alone, so a spot color is sometimes added for a logo.
CMYK on press rewards tight registration and fine mesh. Ask whether your full-color design suits CMYK, sim process or DTG at your quantity. The separation guide compares them side by side.