How to change Android Studio's default build flavor?

With Android Studio 3.5+ you can set default falvors:

android {
  flavorDimensions "stage", "target"
  productFlavors {
    develop {
      getIsDefault().set(true) // that does the magic
      dimension "stage"
      ...

Change the order in which you define them in productFlavors. The IDE always loads the first flavor it finds there as the default.