Thank you! To be honest, it's nothing special or complicated!
I use Fotosketcher (you can download for free to your computer) painting #5 watercolor. I adjust the strength of the affect to how I like and soften the edges. Sometimes I need to adjust the darkness or lightness of an image as well, which you can also do within Fotosketcher.
For the premium packs, I charge a tiny bit because there's more BGs in total (that I edit one by one) that also includes using a different app to manually create the time of day differences.
I hope this was helpful!