Considering the whole concept of how Bitcoins come into existence, Satoshi Nakamoto cannot be just one person, right? It must be a group...
if the whole thing lies in transactions, how could have Satoshi mined the genesis block all by himself?
Satoshi mined most of the first blocks himself as nobody was on the network at the beginning, including the genesis block. You don't need others to mine that one.
Satoshi prepared the basics, published a whitepaper and did the initial coding. Soon after that, he realized he couldn't do all of it alone and found help in the community he build.
So yes, you are right, he didn't build bitcoin alone, there were several developers involved, that are still known to date.