Guardian Angel makes you heal, exhaust, and recall the unit it is attached to. What is recalling? Putting back to hand or base?

Recalling is the action of moving a permanent from its current location (such as a battlefield) to its controller's base.

According to Rule 450 and the associated Patch Notes, a recall is specifically defined as relocating a permanent to its base. It is not a move action (Rule 451), and it does not return the card to your hand.

As noted in your reference to Guardian Angel, when a unit is recalled, it retains its current state (damage, exhausted status, buffs, and attached equipment) unless the specific effect text states otherwise (Rule 453).