Under Device Management ›› Device Trust : Local Domain you would see dtca.crt which is the CA root certificate for the trust network. Same can be found at /config/ssl/ssl.crt/dtca.crt from command line.
BIG-IP systems use the trust architecture to provide a secure framework for configuration synchronization (ConfigSync) and other high availability (HA) features, such as failover for BIG-IP device groups. When the device group components are properly defined, the device group members establish a secure communication channel using SSL certificates to accommodate device group communication and synchronization.
You can visit https://support.f5.com/csp/article/K15664ui more more details