"What app is that?"
I get asked this question at least once a day on Twitter, and it's frankly daft it's taken me this long to just write a note about it rather than answering a thousand people individually.
I use wide variety of different apps and tools for different kinds of output. Including using multiple apps for different parts of each creation.
I'm not wedded to any one of these in particular, they all have their strengths and weaknesses. I'll use whatever fits the task at hand and move work between them liberally.
Here's all the apps and hardware that might be involved in a particular creation...
None of these are affiliate links - I'm just pointing you to official sites for more context
And here's a couple of my common workflows for different styles of illustrations and visuals...