What happens to a unit if you retreat it to HQ?

It goes to HQ in whatever state it was in.

It goes to HQ in whatever state it was in.

Incorrect they are exhausted.

RRG page 13 Retreat.

What happens to a unit if you retreat it to HQ?

Once in the HQ, the unit is no longer part of the battle at the planet it left. It cannot take combat turns against the units still at the planet, but it also cannot be chosen as the defender of an enemy's combat turn.

The unit in the HQ stays in the HQ until the next Command Phase. When you commit your Warlord to a planet in the Command Phase all units in the HQ at the time go with the Warlord to the selected planet. They arrive at the planet exhausted during this step.