Tor ships with a list of directory authorities and some information about them.
In particular, this information includes for each authority its IP address, onion port and onion key fingerprint.
This makes it possible for clients to make an onion connection to one or more authorities for bootstrapping purposes. It then connects to the authority's DirPort ...
If I remember correctly, the Tor software ships with a list of directory authorities. The client has just enough information to bootstrap and then continue to download information about all the other relays in the network.