Instances are created using the openstack server create command, as you can see here:

openstack server create
(--image <image> | --volume <volume>) --flavor <flavor>
[--security-group <security-group>]
[--key-name <key-name>]
[--property <key=value>]
[--file <dest-filename=source-filename>]
[--user-data <user-data>]
[--availability-zone <zone-name>]
[--block-device-mapping <dev-name=mapping>]
[--nic <net-id=net-uuid,v4-fixed-ip=ip-addr,v6-fixed-ip=ip-addr,port-id=port-uuid,auto,none>]
[--network <network>] [--port <port>]
[--hint <key=value>]
[--config-drive <config-drive-volume> | True]
[--min <count>] [--max <count>] [--wait]
<server-name>   

Nova attaches instances to virtual bridges and switches on the compute node via their virtual interfaces, or VIFs. Each VIF has a corresponding Neutron port in the database.

When using the Open vSwitch mechanism driver and Open vSwitch firewall driver, each VIF plugs into the integration bridge on the respective compute node hosting the instance. The virtual switch port is configured with a local VLAN ID that corresponds to the network associated with the Neutron port and VIF. When the iptables_hybrid firewall driver is used, the VIF is connected to a Linux bridge where iptables rules are applied.

When using the Linux bridge mechanism driver, each VIF connects to a Linux bridge that corresponds to the associated network. Every network has a corresponding bridge that is used to segregate traffic at Layer 2.

For a refresher on these concepts, refer to Chapter 4, Virtual Switching Infrastructure Using Linux Bridges, and Chapter 5, Building a Virtual Switching Infrastructure Using Open vSwitch.