Is $lookUp join with main collection or with duplicate collection

I just want to make sure that in atlas search aggregate query if I’ve made dynamic search index on one collection and joins it with another collection with $lookUp is $lookUp uses the another collection from main mongoDb collection or with duplicate atlas search colllection. Does the $lookUp engage my mongodb collection ?

Like I’ve two collections in mongoDb named User and Post and i’ve created search index on post collection and writing aggregate query in which i’ve joined Posts collection with User collection using $lookUp operator Is $lookUp will halt my user collection during it’s search query or it uses the duplicate user collection.

Need help.