You can invoke nRF Util with a modifier using the following command:
nrfutil +<modifier> <subcommand>
For example, the following command uses the "internal" modifier from Adding modifiers to install a internal version of the device command:
nrfutil +internal install --force device
When the command uses +<modifier> to specify the package index, it executes as if you specified the index using a environment variables or command line options.