I'm currently using Ibis Paint X. It's a free digital art program you can get for your mobile device. It has a huge variety of tools, although you need to watch an ad every once in a while to unlock them without paying extra. Still, you can access most of the content without paying a penny!

At the moment I don't make tons of digital art, so a free program like Ibis Paint is more than enough for me. In the future, I may switch to Clip Studio Paint.