Okay, I've done this really quickly, so it's not perfect, but it should get you started.
I'm working in CS2, which is very old, but everything should work the same in later versions. I'm also working with a graphics tablet, which makes everything much easier and tidier - I think the artist of your examples has certainly used one too.
I am focussing on the effect in the first image with these tips.
PS comes with soft brushes, so there shouldn't be any need to download anything, just find the ones that have 0% hardness (or drag this slider to 0 yourself on any brush and see what you get). Choose a large size in the 100 - 300 range - you can also slide the 'Master Diameter' slider up and down to alter the size of any brush.
Next, paint! Create the soft elements you want on a new layer, don't worry about it being a blurry mess, you'll be creating hard edges in a moment.
(I have just done a very rough background under my soft brush layer to give something more interesting to look at behind it.)
Now pick the lasso tool - I have put a little image of the main toolbar to show where it should be (of course this could be different in your version of photoshop). Or you can press L.
Make sure you have the round looking lasso selected, not the other versions (polygonal, magnetic) by clicking and holding the little black triangle in the corner of the lasso button.
Draw your mask shape - basically mark out where you want your hard edges.
One step that was necessary in this example (but I didn't do a screenshot for) was to press control-shift-i to select the 'inverse' of the mask shape I drew. This converts my mask to cover everything
outside the original shape.
(This isn't always necessary, it's just how I chose to do it this time.)
Now press backspace (delete) and everything in the mask will be removed, leaving you with just the hard edge shape with the softer painting inside.
Obviously in the examples you gave, the artist has refined their work a lot more than I have, and played with different variations of the style, like adding finer brushstrokes on top, blurring the shapes with filters like a gaussian blur, etc. They may even have used a different technique - this is simply how I would do that effect, very basically.
The mask will not give a perfectly smooth line either - I would want to spend a bit of time with a hard eraser brush (E) smoothing things off.
You can also just use a combination of hard brushwork (to create a solid, hard edged shape) with varying levels of soft brushwork on top to create the 3D appearance of them, as seems to have been done in the second and third examples you posted. For this, I can only advise you explore the different brushes PS has and play around.
I hope that makes sense, anyway.