Motorcycle helmet is one of the largest US gear queries. For street use the legal floor is DOT FMVSS 218. ECE 22.05/22.06 and Snell M2020/M2025 are additional certifications. A novelty “DOT” sticker on a beanie is not a helmet.
Full-face covers chin and jaw — the usual impact zone in a forward crash. Modular (flip) helmets are popular on US touring bikes; buy ones that stay ECE/DOT certified with the chin bar closed. Open-face and half helmets are legal in many states that do not have a full helmet law. They do not change physics.
Bluetooth is a speaker kit, not a safety rating. Fit matters more than brand: the helmet should be snug at the cheeks with no hot spots after ten minutes. Replace after a crash or on the maker’s calendar, usually five to seven years from manufacture date in the liner.
Shop weights and visor mechanisms in person. Mail-order works if you already know your size in that shell. Do not buy a used helmet unless you know it has never hit the ground.