hello, before i begin to set up my first master/slave site... Is it possible to use all extensions (that are installed in master) in slave site? I do not want to share anything of data between master or slave sites, but i do want to use one joomla installation for different sites. I suppose this is no problem with JMS?
Is it also possible to set up that each new slave site takes over all menus, usergroups, categories, articles from the master site? So the data is copied, but not shared...

Yes you can replicate the master websites to create a slave site.

This kind of replication is illustrated in the tutorial video 0.d
www.jms2win.com/en/tutorial#jms12x-0d

Create a JMS Template rule with
ID = replicMaster
domain = {site_url}/multisites/{site_id}
template site = <master DB>
New prefix : {site_id}_
Deploy directory: {site_dir}

There are other variant but this one is very simple.

ok so far so good. when i look in my cpanel in the file manager, i find a map multisites and submap 123456 (my site id). when opening the submap i see again the joomla directories of my master site. this is normal? is this because of symbolic link? when editing some files in this map, i am editing the master files?

Yes the objective of the symbolic link is to give you the same kind of directory structure in each website. So this is absolutely normal that you see and share the same thing. This is the concept of JMS. Share the same joomla installation (PHP code - so files and folders).

ok, i begin to understand... But one more question about this: i created a slave site and after this i added a new categorie and article to the master. All new slave sites will have these new articles, but the sites that are created before not. what is the easiest way to add this new article/categorie to the first slave site? do i have to run a sql command in phpmyadmin? or are there joomla extensions that make it possible to export and import content/categories from one site to another?

See the "Article Sharing for JMS" that allow reading the articles from any websites.
So you can share the articles.
www.jms2win.com/en/joomla-multisite?page...47&category_id=1