If you want to make app mockups, ads, logos, signs or graphics for the web, take a look at Graphics by Autodesk.
Vector graphics is great for many things. The big advantage over image formats like .jpeg, .png and .gif is that it is scalable without losing any quality, and file sizes are usually very small. The downside is that it is really bad at handling photographic images.
In Graphic you combine text, lines, curves, circles, rectangles and other shapes into images. You can combine them in infinitely many ways with different colors, gradients, opacity and shadows.
Graphic works with your fingers or styluses, including Apple Pencil. I most use my fingers but occasionally Apple Pencil as well. Graphic has a loupe function that lets you see what's under your finger. This is great when you want to make precise adjustments.
There is a learning curve drawing vector graphics. You can easily get started drawing shapes, but after a while you will notice that there are many things to learn to get really good at it. If you have experience working with other vector graphics editors, like Adobes Illustrator, Graphic will be no challenge to you.
I have used Graphic for years and like it a lot. I am not a professional designer though.
The app is priced at 9 USD in the App Store. You may not want to pay that for something you don't really need, but if this is something you work with, the price is no problem. There is a macOS version as well and all your work is synced between devices via iCloud.
(The Savvity logotype was created by Jesper Wahlström)