How to configure a dynamic VPN.
Officially these are the solutions for a Dynamic VPN deployment in SRX devices:
https://www.juniper.net/documentation/us/en/software/junos/vpn-ipsec/topics/topic-map/security-autovpn-on-hub-and-spoke-devices.html#id_yrz_vwj_fpb
https://www.juniper.net/documentation/us/en/software/junos/vpn-ipsec/topics/topic-map/security-auto-discovery-vpns.html#id-example-improving-network-resource-utilization-with-auto-discovery-vpn-dynamic-tunnels