My 3 go-to app this year, after I give up a phone for a tablet, are MoonReader, Paperpile and Google Colab. MoonReader for book reading (lots of great textbooks this year on data science), Paperpile as reference manager (and pretty nifty way to share too), and Colab for coding on the go.
The other essential tools are a good fountain pen, a good notebook and a bottle of water.
The other essential tools are a good fountain pen, a good notebook and a bottle of water.