Requirement of work or school account to access MS365 admin center

I am building a proof of concept app and trying to fetch data and intelligence from the world of MS using graph API, (Java SDK). Initially I thought all I have to do is build a OAuth2.0 client for MS and its done, but there is more to it, if you want any thing more than just the name of user. As I am trying to get the user's calendar data, (and not just the name as mentioned on tutorials) I came to know that we need the said user to have an O365 license. For testing purpose I created an outlook email account which is working just like standard email (like Gmail), thus same is having Exchange online in the back-end as per my understanding, and used the same account to setup Azure active directory, and OAuth2.0 client app registration but I couldn't fetch calendar events, all I am getting is "Resource not found" error.

After some research I found out that we need to add an O365 license to the said user, using MS365 admin-center, but to access the MS365 admin-center the MS system is requiring work/school accounts, a standard outlook email, I created for testing purposes is not letting me in. On reading the calendar API part of MS-graph I don't see the requirement of licenses, or work/school accounts. Could you please through some light on how to get more worthy data using graph and what all licenses, or anything else needed for the same?

Other than above I am very confused on how to use MS-graph API to get user data when user is not in my tenant, as obviously user is already using MS services (say outlook calendar) and thus must be present in some other tenant, and now want to get that data (say events of calendar) in my app, as the same user is using my app as well? This is classical case for which OAuth2.0 developed but MS have its own things like tenants, licenses, work/school accounts, which I appreciate but could you please let me know what all is needed, I would really appreciate if you can guide me to the right part of documentation.