Those are freehand I use Gel polish when I hand paint details, I find it easiest. If you want to give it a go on some nail pops, you'd have an easier time if you first sketched your design on paper for practice, then sketched it lightly with a pencil on the nail itself (buff suface), then go in with black gel polish - and use a very thin and short fine detail brush for the black. I cut my own brushes, I buy cheap detail brushes, then I cut them so they're shorter and only have 3-5 strands on them lol.
GetBuffed Nails (sarah elmaz) handpaints Disney characters with Gel polish too, but she's a godess at hand painting, I feel both inspired and intimidated by her at the same time. Check her out, she sometimes gives some good advice how to achieve details, and she's holding a class in Europe in August which is great for wanting to improve any skill.