Its called a torque converter. Simplest way to explain it is that its an automatic clutch that engages/disengages the transmission. At low rpms with the brakes pressed it slips so the transmission sits idle. They are designed to require certain amounts of torgue to engage the transmission to make it work.

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there is a slip mechanism called a torque converter, that at low speeds, works like a fluid coupling. that's why the vehicle creeps forwards when you ease off the brake.

think of a fluid filled donut with an impeller driven by the engine, and another impeller driving the transmission. at idle speeds the system has some slip. at higher speed differential there is torque multiplication due to a stator inside redirecting fluid flow. but that's well beyond ELI5 without diagrams and videos.

manual transmission cars don't have a fluid coupling, but rely on a friction clutch that works much like a brake pedal but inversely. stepping on the pedal releases the clutch into false neutral. and releasing the pedal engages the clutch and lets you bark the tires and get going.

In addition to torque converter, many higher end sport cars use Dual Clutch Transmissions. In that case there is an actual clutch that engages/slips into 1st gear, just like you would do in a manual.

The gear itself doesn't wear at all from static force, since it's just pieces of metal meshing with each other but not moving. They can wear a bit from turning under power, though this is pretty negligible in vehicle transmissions that are working properly.

I think you're thinking of clutches. Manual transmission cars use clutches which physically slip to start from a stop. If you left one partially engaged while braked at a full stop, it would indeed wear very fast, so don't do that. Automatic transmission cars use torque converters, which as described in the other posts is a fluid coupling instead of a mechanical slippage, so it doesn't wear significantly from exerting force while the car is held at a stop by the brakes.