WP 5.x : Where to find the Collapse-Expand button in the Editor ?

This is an answer to a previous question on this topic.

In order to find the “Classic Block”:
Create a new paragraph by clicking on the page
You should find a plus sign in a circle to the right; when you hover over it, the hover text is “Add block”.
Click on this plus sign. A graphic menu will appear.
Either click on the icon labelled “Classic” (type “Classic” in the search box if you don’t see it right away).
Once you have selected the “Classic Block”, the Collapse-Expand button should appear.