Bitcoin is a product of previous digital money that has been created decades of years before the existence of bitcoin. And as far as I know, it all started from Adam Back's Hashcash to Wei Dai's bmoney up until to Nick Szabo's Bitgold

Satoshi combined combined these founding pieces of technology that results to an innovated cryptographic based currency called bitcoin.

Full details here - https://cacm.acm.org/magazines/2017/12/223058-bitcoins-academic-pedigree/fulltext?mobile=false