How do they film the commercials that show previews of "LIVE" TV shows!

Question: How do they film the commercials that show previews of "LIVE" TV shows like so you think you can dance!?
Answers:

Dress rehearsal footage from the upcoming show or clips from earlier episodes are used in promos for such shows!.Www@Enter-QA@Com

it's usally there dress rehershialWww@Enter-QA@Com

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