To reduce the number of BGP sessions in an IBGP network, you can use a route reflector, which acts as a focal point for IBGP sessions and readvertises the prefixes to all other peers. To configure a route reflector, you need to enable the route-reflector-client option on the neighbor-group settings of the hub device. This will make the hub device act as a route reflector server and the other devices as route reflector clients. References := Route exchange | FortiGate / FortiOS 7.2.0 - Fortinet Documentation

C). The hub adds routes based on IKE negotiations: This is part of the ADVPN functionality where the hub learns about the networks behind the spokes and can add routes dynamically based on the IKE negotiations with the spokes.
D). You must configure phase 2 quick mode selectors to 0.0.0.0 0.0.0.0: This wildcard setting in the phase 2 selectors allows any-to-any tunnel establishment, which is necessary for the dynamic creation of spoke-to-spoke tunnels.
These configurations are outlined in Fortinet's documentation for setting up ADVPN, where the hub's role in route control and the use of wildcard selectors for phase 2 are emphasized to enable dynamic tunneling between spokes.

From the OSPF interface command output, we can conclude that the port3 network has more than one OSPF router because the Neighbor Count is 2, indicating the presence of another OSPF router besides NGFW-1.
Additionally, we can deduce that the interfaces of the OSPF routers match the MTU value configured as 1500, which is necessary for OSPF neighbors to form adjacencies. The MTU mismatch would prevent OSPF from forming a neighbor relationship.
References:
* Fortinet FortiOS Handbook: OSPF Configuration

To use the VPN interfaces in a policy, you need to install the VPN community and gateway configuration on the FortiGate devices first. This will create the VPN interfaces on the FortiGate and sync them with FortiManager. References:
* Creating IPsec VPN communities
* VPN | FortiGate / FortiOS 7.2.0

BFD (Bidirectional Forwarding Detection) is a protocol that provides fast failure detection for BGP by sending periodic messages to verify the connectivity between two peers1. BFD can be enabled on both connected FortiGate devices by using the command set bfd enable under the BGP configuration2. References:
= Technical Tip : FortiGate BFD implementation and examples ..., Configure BGP | FortiGate / FortiOS 7.0.2
- Fortinet Documentation