Q: How do you fill a perfume atomiser?
A: Most refill by pressing the atomiser down onto the nozzle of the original bottle with the cap removed, and pumping several times. Do it slowly so the chamber fills rather than spraying.
Q: How much perfume does a travel atomiser hold?
A: Enough for a few days away, which is the point of it. Taking a decanted amount also avoids risking the full bottle in a bag.
Q: Can you take a perfume atomiser in hand luggage?
A: Yes, provided it is within the liquid allowance for your airline, which is what makes a small refillable atomiser useful for travel.