What's the role of Python in ArcGIS?

There are 2 main parts in ArcGIS ecosystem that involve Python: ArcGIS API for Python and ArcPy.

In short, ArcGIS API for Python is mainly for writing code to build a data pipeline that work with cloud and web GIS, while ArcPy enhances customizability of traditional desktop GIS such as ArcGIS Pro, so you can glue various tools and steps together with Python script, instead of in UI.

With regard to your question, if you aren't working with Pro, it is likely that you'll build with ArcGIS API for Python. There are some pretty comprehensive documentation on our developer website.

If you have question feel free to reach out on our forum.