Basics of using notebooks with SageMath

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I'm a seasoned user of Mathematica, just moving to SageMath†. I get SageMath running under Linux (under Mint Vera, after installing sagemath 9.5-4 and automatically selected dependencies including jupyter-core 4.9.1-1), and can use SageMath in the terminal. I'd want to use jupyter, or whatever would give me the comfort of notebooks.

When I do sage -n jupyter I end up in Firefox on page localhost:8888/tree showing me 3 tabs Files/Running/Clusters. In the Running tabs I'm told there are no terminals or notebooks running.

How do I make a jupyter (or other) notebook, and what's the local equivalent of the enter key that lets me run a notebook cell ?

† I long wanted to go open-source. The need for Schoof–Elkies–Atkin point counting was what triggered the switch.