There are several steps to configuring a full-mesh site-to-site VPN.
Every gateway must be configured with a tunnel to every other gateway.
For information about what address objects are needed, you can review the example: Example: Full-Mesh Site-to-Site VPN
IPsec policies are selected when configuring an IPsec VPN connection. You can use the Default IPsec policy, use another existing policy, or you can add a new IPsec policy.