What happens if star crossed the nonambushed unit in reaction to ambush while I’m the defender

Your pre-Ambush unit gets bounced, but your Ambush unit stays — and you keep the battlefield and your Defender designation.

Star-Crossed requires one friendly and one enemy unit, so the attacker is bouncing their own unit alongside your "non-Ambush" unit. Note the timing trap: there is no reaction window to the Ambush play itself (units resolve immediately) — Star-Crossed can only go on the chain in response to the Ambush unit's enter triggers, or once the state opens and the attacker has priority. Since it resolves first (LIFO), both chosen units return to hand before anything else.

After it resolves you still have your Ambush unit at the battlefield and a Combat is ongoing, so you don't lose control and you keep the Defender designation (Rule 187.4.c). What happens next depends on what the attacker chose as their friendly target: if they bounced their own attacking unit (the usual play), no combat damage occurs and at Combat Resolution you're the only player with units — you win the combat and control stays with you. If they bounced some other friendly unit instead, your Ambush unit remains to fight their attacker and combat proceeds normally.

FAQ #10057; Rule 187.4.c, 465.1, 466.3.