How do I get this wavy effect on lines?

Hello,

In the attached picture you can see a portrait of a woman from behind. If you look closely you can see than the lines that makle up her head/hair are lines which has thicker and thinner parts creating that kind of artsy uneven wavy look. I am feeling pretty confident that it wasn't drawn like that from the beginning, but rather some kind of technique or filter was used. Or some special brush maybe

Either way, I can't seem to find the answer to how this was done. I have been trying to recreate it both in procreate and photoshop without success.

Anyone have any suggestions how I can get this effect on a line, or how he/she did to get this look?

Thanks