Apple branded hdmi dongle. About 50 buck's , at least they were . One thing I can tell you is Do not use the off brand 18-30$ look alikes as they don’t work or won’t work for long. The Apple system won’t recognize them.. I know this from personal experience.
Look for Lightning digital av adapter on apples site to know what it is. While there if just order it from them .
Using the dongle it uses your phones high speed data . The hotspot data I can guarantee you isn’t really unlimited and you’ll be cut off very quickly.