I have installed the PyXLL add-in but I don't see the "Insert Python" button in the "Formulas" tab in Excel.
With PyXLL, you can import code that is written outside of Excel (e.g. in VS Code or another Python IDE) and do not need to insert Python code directly into your workbook.
The "Insert Python" button shown above is part of Microsoft's "Python in Excel" feature, not PyXLL. It is fundamentally different to PyXLL, and requires all Python code to be written directly in the workbook and is run on the Azure Cloud.
PyXLL, in comparison, uses your own local Python environment where you can install any packages you require, and can import code that is written outside of the workbook. This allows for proper code version control, as well as sharing/re-using code between workbooks.
For a more detailed article about the differences between PyXLL and Microsoft's "Python in Excel" feature, please see What are the differences between PyXLL and Microsoft's "Python in Excel"?
If you are new to PyXLL then this video will help get you started
To use a Jupyter notebook inside Excel, please see this video