What is real and what is not?!

OK, so this may be a stupid question but here goes...

Either the casting of this film is very good, or they use a lot of archive footage. Sometimes it is nearly impossible for me to see whether there are the real Beach Boys or not.

Some of the interviews I have seen in other shows, so I guess they are authentic. But what parts are made afterwards, and what parts is from their actual performances?

Sorry if this IS a stupid question, but I really want to know!

Best regards,

Tobias
Sweden