Isolated Selected Group

[MahaB82A]

is use of Isolated Selected Group command necessary in this video [t: 0.56]? because everything can be done with ungroup. Link video follows. 

Sparkling Stars

[Monika Gause]

I would recommend that you get a good book. Classroom in a book or Real World Illustrator (yes it's old, but it's still good). Then read that. Just diving into random videos will probably not help you with your learning.

As for that video: I don't think isolating makes a lot of sense in his workflow. He could have just ungrouped the thing.

Did you follow along with the presenter? Did you actually do what he did? Because then you should notice if something is necessary.

[MahaB82A]

Only thing I learned form this video is using Flare tool. Which is new to me.