Basic Authentication will end on October so I decided to make a guide on how to handle it

BetterLicenses 

Many thanks for this help.   It begins to be clearer but still murky. Still better than Microsoft """explanations""".

So now, I followed your process and discovered that three users (myself included) and an automatic mailbox use the simple authentication. I see it is with "Office 365 Exchange Online". And now what ? I can't manage to understand what is wrong and how to fix it.

Thanks for any more explanations.

edit :

I think this is this point I do not understand :

"After providing instructions to users on how to migrate their devices to Modern Authentication your tenant should prevent those users from adding new devices through legacy authentication.". The tenant (me) does not know how to migrate the devices and not even what devices