Base code of custom transformer models not managed by Huggingface

Hello everyone, I am fairly new to Huggingface and Transformers in general. I am trying to analyze the text simplification approaches on custom models that are not managed by Huggingface but uploaded individually, such as:

I am failing to see the base code for encoding and decoding and hence the simpification mechanisms.
How do I reach the full source code?

Many thanks,