A website is composed of 2 parts:
1) PHP code of joomla and extension
2) Database where the website content is stored.
With JMS multisites, you share the PHP code - the files and folders on the disk.
By default each website has its own content. That mean has a speciific DB or at least a specific table prefix when using the same DB for several webiste.
Therefore when you create different website, by default, all the DB content is specific for each website. You need to specify to JMS Multsities when you want to share MySQL table between websites.
So the event but also the user, the articles, forum or whatever that is stored in DB is specific to each websites.
Concerning the update/upgrade see the chapter 8 in the user manual.
You have 3 cases
1) Update of Joomla itself - This only concern the PHP code and not the DB
2) Update of an extension that does not impact the DB layout
3) Upgrader of an extension that impact (change) the DB layout. In this case you have to re-install the extension in each slave site to let the extension perform the upgrade.
When there is only PHP code update without any DB impaction, you can update the extension only once as the PHP code is shared between all the websites.
Here it is the list of extension currently defined in our JMS tools and also the one that can be shared
www.jms2win.com/component/content/article/135