Plugin activation options

Just another question I would like to get clarity on:

So with mu-plugins folder plugins are automatically activated for all sites and plugins folder it is on a blog-by-blog basis.

However, when you upload a plugin to plugins directory you still get to either activate it or activate it sitewide.

Does this mean if you just activate it, it is available only for main site only and if you activate it sitewide it is available for all sites?

The thing that is getting me confused is that BuddyPress is activated sitewide, which makes me doubt what I concluded to be the differenc