BitSet internally uses long array of size 6.
But it can contain 2^31-1 bits.
long = 64 bit
6 longs = 64 * 6. But is much less than 2^31-1. Please explain this trick.
The internal array which stores the values expands if necessary, just like an ArrayList would do for objects. Your statement is therefore false.
There is a hard limit of 2^31-1 (bits) for the size, but that's only because the operations (set, flip, etc.) take the index parameter as an int. Which is probably big enough for most use cases.