How does “Query DNS upstreams in parallel” technically work while I don’t have a way to set to use multiple DNS within AdGuard?

I'm one of the new people who got the AdGuard discount and I want to fully utilize my purchase

Also, AdGuard should really start allowing people to use multiple DNS addresses for flexibility and allow DNS cache management like the ones from Technitium's DNS Server where I actually tried using first before I knew the AdGuard discount