How VDOM-DNS works

I'm referring two KBs below for this issue:
https://community.fortinet.com/t5/FortiGate/Technical-Tip-FortiGate-alt-primary-alt-secondary-DNS-server/ta-p/275269
https://community.fortinet.com/t5/FortiGate/Technical-Tip-Configuration-per-VDOM-DNS/ta-p/190815

But in reality with 7.0.13, the vdom-dns config accepts only alt-primary/alt-secondary unlike the 2nd KB describes.
With this, how is the DNS decided at the vdom (test-vdom)? Always ask global primary/secondary first? Then only when they're unreachable vdom-dns is used? Or only vdom-dns is used? I prefer the latter behavior but not sure.

Also, what protocol would be used if alt-primary/alt-secondary was chosen? Same as the primary/secondary?

fg40f-utm (global) # config sys dns   fg40f-utm (dns) # get primary             : 96.45.45.45 secondary           : 96.45.46.46 protocol            : dot ssl-certificate     : Fortinet_Factory server-hostname     : "globalsdns.fortinet.net" domain              : ip6-primary         : :: ip6-secondary       : :: timeout             : 5 retry               : 2 dns-cache-limit     : 5000 dns-cache-ttl       : 1800 cache-notfound-responses: disable source-ip           : 0.0.0.0 interface-select-method: auto server-select-method: least-rtt alt-primary         : 0.0.0.0 alt-secondary       : 0.0.0.0 log                 : disable   fg40f-utm (test-vdom) # config system vdom-dns   fg40f-utm (vdom-dns) # get vdom-dns            : disable alt-primary         : 0.0.0.0 alt-secondary       : 0.0.0.0

Toshi