If an app cannot avoid complex queries, can it still scale?

I have chosen MongoDB for the database layer of my APP.

Even after implementing indexes and sharding, the queries that are generated by the users of this app will require multiple trips to the Mongo Server, multiple collections, etc.

Can this app still scale to 100s of millions or even billions of users?

I cannot imagine a large successful app that doesn’t have to make multiple trips to the database - this is true for all databases I’m familiar with, so I think your concern is maybe slightly misplaced - the key is not making any unnecessary extra calls to the database, and most importantly making sure that all the operations in the database are as efficient as possible (meaning, well indexed, etc).