Can I work with larger numbers than an unsigned long long in Arduino?
There are many ways to use numbers bigger than an unsigned long on Arduino. The right one depends on the application.

Floats and Doubles
floats and doubles types on Arduino can hold exact integers up  2^53. That is a very large number. They are built into the Arduino compiler. 

uint64_t
The uint64_t type can hold exact integers up to 2^64. That is a very large number. This type is built into the Arduino compiler, but note that some Arduino library functions do not have versions for this type so may need to down-convert for printing and serializing and stuff like that. 

BigNumber (or bc)
The BigNumber type can represent exact numbers (integer or otherwise) limited only by available memory- but certainly hundreds of digits long on Arduino. These are extremely big numbers. This type is not built into the Arduino, but there is a nice Arduino library version available... 

https://github.com/nickgammon/BigNumber

Again, this type is not supprted by the Arduino built-in library functions so  you will need to either down-convert or find BigNumber compatible functions for things like printing and serializing.