Thanks tbp1!! Much appreciated!!!
I'm french so I may have difficulty expressing myself properly, so bear with me ;)
I gather that I might not have explained properly what I mean by 3 ways to start the server. ( i do not mean the 3 options in the debug dropdown! Those '3' choices are just a confusing coincidence!)
What I mean by that is that those 3 ways have completely different 'settings' applied (such as what environment, what working dir path, what string to start python with 'py', 'python' , 'python3', etc.) and may not necessarily be setup in advance for your python and /or vscode installations on your machine:
1- (for experienced python programmers who want to debug the leoserver live in vscode) Starting the server by/with vscode (choose "server and extension" in the debug top dropdown.) means that python is setup and started by vscode. As-if the leobridgeserver itself was the python project that vscode tries to run and debug , So it will use the vscode settings for running and debugging python that you have set in your vscode settings. (not leointeg's settings) leointeg has no control, so this is for people who have already setup their vscode to run/debug with python.
2- another way, is in another external terminal like you did . (settings are all controlled by what you type to start python and the script - if it worked, take note of it and copy it in leointeg's settings panel so it can start it itself with the proper command to start python on your machine.)
Screenshot below is vscode's run/debug choices. this is not 'leointeg's options'. Choose 'run extension' to have the real "1.0 user experience".
3- Third (and best way) is by having leointeg start the server itself. so in the debug top dropdown choose "Run Extension" (then start leointeg with F5, then in the new vscode window that opens i.e. the 'extension host' goto the command palette and start typing 'leo welcome' to access leointeg's settings page.) you can enter a specific string to start python if the defaults don't work on your machine. (That's where noting what worked by trying in an external terminal might come handy!)
You don't have to necessarily have a server started beforehand. Leointeg has buttons to start and connect to a server in it's outline pane while not connected to a server.
Please zoom in the parts noted in red to see what I mean.

Leointeg has a special 'welcome/settings' screen where everything is customizable and has somewhat helpful explanation text beneath each option.
Note: leointeg settings only exist/make sense in the extension host window that was started with F5 while it's in development. (So yes, its kind of counter intuitive to have to start the extension before going into its settings to, say, change the python command string for the server, etc.) Once released as a real extension the options and the settings panel will exist all the time in vscode.
Now, when leointeg tries to start the server, either automatically at startup if you checked the 'start server' option, or, with the 'start server' button in the leointegration panel, you will see what command leointeg tried to use in the output pane below. (see text circled in red below).
thats it :)
Also : What do you mean: 'F5' on an @file? This does not make any sense to me at all?!?
F5 is a vscode command to start running and debugging the main project in it's workspace that the workspace is configured to do. So unless you have cursor focus in an opened file, i.e. the actual file derived by the @file node, not the body pane of the @file, ...and that vscode considers that file in its workspace etc... then F5 will start it in the correct interpreter/compiler that your vscode is configured to use for a given language in a given workspace.
So thanks a lot to tbp1 and to every one who would like to give this a try! and don't hesitate to ask questions!
I realize now that the instructional video about how to run and start the development version had no other choice than 'run extension' at the time. since then I've added 'run server' and 'run both', which trips most people. I should make it more explicit to choose 'run extension' in vscode's debug view to have the final, true experience of a (soon to be released )1.0 version.
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Félix