What data is stored within the server?

Is there a minimongo-like in-memory database within meteor server or are mongo logs directly streamed out to subscribed clients? In addition to publish/subscribe info, what else is stored within the server? Is there anything that explains these under-the-hood issues?

you meteor app listen 3000 port by default, and you mongodb listen 3001, try to type in console

mongo --port 3001
use meteor

and you can see data of meteor app

Is this true and especially the last third ?!?

“Meteor Livequery is a family of live database connectors. These connectors let you perform “live queries” against your favorite database: queries that not only return the result of the query at the time it is made, but that then go on to return a stream of create, update, and delete messages that inform you of any changes to the result of the query as time passes.” - https://www.meteor.com/livequery

Yes. This is exposed via the APIs: