Hey there, I think this wowhead article does a good job of explaining how the system works: https://www.wowhead.com/guide/item-upgrade-system-dragonflight
As for adventure gear, it mostly upgrades with either flightstones and whelpling crests. If memory serves, I think you can get that gear from rare mobs and running heroic dungeons.
Oh no, not urging! That’ll stop them this time.