The edges are sharp and the resolution is high because these were made in a vector graphics program, namely Inkscape. Vector graphics are composed of lines and shapes, not pixels, meaning I can export them at whatever resolution I want and the image will always remain perfectly clean. However, vector art is incredibly time-consuming, and smaller details can be impossible to work with since they're all essentially on their own layer. Because of this, I've since saved up for a drawing tablet and now work with Photoshop.
Thank you for your feedback! - ^_^ -