A plate carrier.carries armored plates. Different plates can stop different calibers of bullets. Some cheap ones are made of steel, which are almost never recommended because they're heavy as shit and the bullet fragmenting off the plate will guillotine you. Higher quality ones are made of ceramic. Those are most common. Then in pistol caliber rated ones they're even made of just polymer.

A flak jacket I believe would really only offer protection from explosive fragments.

I suspect Elon is wearing a plate carrier with a ballistic plate to protect his bib area lol it's either a shitty little steel one or a ceramic that is flat and lacks any curves. It looks so ridiculously shitty that I wanna say it's the former.

Bib for eating lead lol