No, you actually don't. You gain knowledge that some of the code is maybe publicly available, just like linux. The rest, however, is often still kept secret, legally or not, and has been since time immemorial.

You don't have to take my word for it though, go ask bradley kuhn if he thinks that vendors have a tendency to comply with the GPL for the kernel :)

Do you though? Google, as the liscenser is free to keep portions of fuschia under lock and key, so that's not really the case.