From the docs:
When the dataframe has grouping variables, operations are performed per group
You can think of this as Explorer creating sub-dataframes using the grouping variable and applying operations on each group. But importantly, the sub-dataframe does not contain the variable any more (since it does not vary) and the variable cannot be used any longer.
┌───────────────────────────────────────────┐
┌──────────────────┐ │ Country 1 Country 2 Country N │
│ │ group_by │ ┌─────────┐ ┌───────────┐ ┌──────────┐ │
│ Fossil Fuels ├───────────► │ Sub-DF │ │ Sub-DF │ │ Sub-DF │ │
│ │ │ │ │ │ │ │ │ │
└──────────────────┘ │ └─────────┘ └───────────┘ └──────────┘ │
└───────────────────────────────────────────┘
