Hopefully, this is a quick answer -- I have the EFR32xG21 development kit with the three boards and would like to reinstall the default applications so the boards work just as I received them out of the box. But I can't seem to find the firmware anymore.
You can find that example in the SDK. Studio launcher shows contextual content so to make sure the example is visible make sure to select a radio board with EFR32MG21 (this can either a connected kit or a manually added part under "My products" on the lower left corner) and the most recent Bluetooth SDK (v2.12.0).
This is how you would select the radio board (I don't have a kit with me so I have manually added it):
And then by selecting the correct SDK on the top side of the launcher view you will see the example available:
Now all you need to do is create the example, build and flash. If the example doesn't run as expected then the most likely explanation is that you are missing a bootloader. The easiest way to flash a bootloader is to flash one of the pre-built demos from the demo column, for example the soc-empty.