You can select which theme and plugins are used with which site. In other words, there will be only one copy of each theme and plugin on your WordPress Multisite installation, but you can still pick and choose which ones you want for each site.
No, each site can still use a different theme. However, every site that uses the same theme will be using the same version of the theme, as there will be only one copy of the theme files, shared by the whole installation.
Hope it helps.
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