I often need to use a text editor while writing code to paste random notes but especially JSON responses, where I format them using a plugin (for Sublime).
I recently heard about the 'scratch file' feature in IntelliJ / Android Studio which does exactly what I need it to - except I can't make it format JSON I paste in nicely.
How can I make Android Studio format JSON in a scratch buffer file?
You are asking about two seperate things: scratch files and scratch buffers.
When you create a scratch file in IntelliJ you can choose the type of the file (e.g. JSON) that you want to create. Based on file's type, IntelliJ provides code formatting (use Code->Reformat code).
However, scratch buffers are just simple .txt files and the only formatting that can be used is the one associated to .txt format. So, if you put JSON into scratch buffer it won't get formatted with JSON type formatter.
I would encourage you to use scratch files instead of scratch buffers if you want JSON formatting.
More information can be found at IntelliJ's official page https://www.jetbrains.com/help/idea/2016.2/scratches.html.