Warforged and Constructs: If they are destroyed, are they destroyed for good?

Warforged, my player insists that because he is not biological in nature; poison, lack of oxygen, drowning, etc. doesn’t affect him. Is this true?

“I ready to cast Catapult on a projectile coming within range” would you be able to interrupt a ranged attack this way?

Can a Bag of Holding be opened from the inside?

Grabbing an arcane focus from belt counts as your 1 free object interaction or is a part of casting action?