There is more than one place where we have a helper method called GetMongoQueryProvider which is used to find the MongoQueryProvider from the inner parameter to one of the Join methods.

Replace it with a single extension method GetCollectionInfo that returns two values like:

var (innerCollectionName, innerSerializer) = innerExpression.GetCollectionInfo(containerExpression: expression);

The reason for the name change and the return value changes is that we don't actually care about the query provider, the query provider is just an intermediate step in getting the underlying inner collection and serializer info.