For vector art there’s Adobe illustrator (paid), Sketch (paid), or inkscape (free)
For a photo editor style app there’s Photoshop (paid), Affinity Photo (paid), gimp (free), paint.net (free), Pixlr (free), or photopea (free)
For a pure art program there’s tons. The free ones I know of are FireAlpaca, MediBang paint, Autodesk sketchbook, ArtWeaver, and Krita.
The paid ones I know of are Clip Studio Paint, Rebelle, ArtRage, and Paint tool sai.
If you want to make simple, minimalist Ui designs with little effort, Google Drawings is pretty neat.
Here’s a link to a helpful tutorial for it An Overview of Google Drawings: UI Design Made Simple (100% Free to Use)