I’ve found the fastai docs website to be very helpful in answering questions like this - I’ve had similar and still am learning more about the language model learner. Take a look at https://docs.fast.ai/text.learner.html#language_model_learner for high level details on how the language_model_learner works.
The function definition says that pretrained defaults to True and pretrained_fnames defaults to None. When looking at the source code for the language_model_learner available here:
It shows that if you don’t pass in a pretrained model, one is loaded by default (if you are using the AWD_LSTM or Transformer architectures.)