Ah, okay. You're wondering how they alter the look of the default models? I'm guessing you're referring to games like Being A Dik or Summer's Gone?
Those are made with a free software called Daz. Who hosts a marketplace for models, hairs, clothing, or any number of other assets that you can buy (or pirate). So, for example, you buy Victoria 8 from the Daz Store:
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. Devs don't want to use a bunch of the same models, so what you can do inside of Daz is get another figure (of the same generation/number. So, if you bought Victoria 8, you'd need to get another figure with an 8 in the name. Olympia 8, for example.). Once you have two figures, you can lower the sliders and mix the shapes of both figures. If you need fine-tuned morphs for certain parts of the face or bodies, you can use morph packages:
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.
Here's a pretty solid basics playlist for Daz:
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, Parmy Badham (spelling is probably wrong.), IT-Roy, and Game Developer Training all might be worth a look as well.