Let’s just postulate that somewhere on Earth is a extremely rare mineral called Vibranium, which, when combined with an experimental iron alloy, creates a virtually indestructible material that can absorb and deflect pretty much any level of force, leaving the person behind it completely unharmed. It still doesn’t explain how Captain America can bounce his shield off multiple enemies at incredible speeds and have it come right back to him as if he’s performing the world’s deadliest pool trick shot.