Database structure of the class sever

As I have seen a lot of collections inside a single database. Would it be a best practice to make a separate database for each collection? Does it make any difference in the performance of an application?
Just curious to know. I saw someone doing this.

It’s not a best practice to have a separate database for each collection. Its difficult to manage every database, you may lose functionality like lookups as they require data to be on the same database.
Also, it’s very hard to optimize the data at the beginning without knowing what the bottleneck is.