ADVPN Cross Regional Spoke to Hub Shortcut

I'm lab testing a few different ADVPN setups and noticed the Cross-regional spoke to hub shortcuts:

https://docs.fortinet.com/document/fortigate/7.6.0/sd-wan-architecture-for-enterprise/242856/using-ibgp-between-regions-with-inter-region-advpn

The docs are missing at lot of details so I'm wondering if anyone knows how this dynamic tunnel from Branch 1 (Region A) to HUB in region B is created? Which tunnel interface is used on Region B HUB.

Does the static VPN tunnel between the hubs need any ADVPN specific config rather than just standard static tunnels?

I have dynamic tunnels working ok within the same region so I have a basic understanding of how the tunnel is formed. Just not clear on when it comes to Multi region, specifically the SPOKE to the HUB in the other region.