There are two ways in which instances without public addresses can make use of the Internet.
Those instances that are without public IP addresses can route their traffic through a NAT instance or a NAT gateway so that it can access the Internet. In order to traverse the Internet, these instances make use of public IP address of the NAT gateway or the NAT instance. Outbound communication is allowed by the NAT instance or NAT gateway but it do not permit machines on the Internet to start a connection with the addressed instances privately.
For those VPCs that are provided by a hardware VPN connection or direct connect connection, the instances can route the Internet traffic through the virtual private gateway to the existing data centre. It can then access the Internet through the existing egress points and also new tweak security or monitoring devices.