Do Hong Kong and mainland China have the same currency or use the same money?

No. China uses the CNY/Yuan. Hong Kong uses the HKD/Hong Kong dollar.
The rate CNY-HKD is roughly 1-1.2

no they have different currency

no. hong kong has its own currency called HKD (hong kong dollar) and the mainlands has RMB (ren men bi). however, in most places in hong kong, the exchange rate is the same or in some cases HKD:RMB will be 1:1.25

Different currency. China uses yuan, hong kong uses Hong Kong dollar.