Hi @ooumi77 ,
Here are the following answers to your queries:
(1) POST /me/events - Is used for Delegated Permissions which can be used using Graph Explorer, the access token you get here is on behalf of your account and the selected permissions only apply to your calendar.
(2) POST /users/{id|userPrincipalName}/events -
To get access to a colleague's calendar you have to acquire an access token on their behalf or read/write to a shared or delegated calendar from them. To perform this it requires application scope.
I tried to replicate the same at my end. please find the details below.
Hope this helps,
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