How do you "target" emotes

Hover over the circle indicating the emote over the desired player and press F in order to respond to that specific command. F is by default the "Understood/Agree" command.

You can "agree" with almost any voice line, including Ultimate Status, Health Needed, Hellos, and Voice Lines.

In addition to Retrosaur's answer, if you hover your crosshair over another player (even when they don't show the emote icon) and execute an emote it will be directed towards them, similar to your example.

PlayerX says hello to PlayerY

You see your second example when an emote is executed with no player in your crosshair.

Me says hello