Putting the car in gear connects the engine to the transmission. An engine cannot be running and not spinning so even if you’re not pushing the accelerator the idling engine will roll the car if you’re not applying the breaks.

Edit: brakes not breaks. Ironically I answered a question about the difference between the two a while back lol.

In automatic transmissions, there is a torque converter. This is like two fans, one connected to the engine and one to the start of the transmission. This allows the car to sit still while the engine moves.

As long as the engine is moving, the fan is blowing oil at the other fan, and so it's pushing the car forward. When you step on the gas the fan blows harder and so the force forward is stronger.

Fluid linkage in the torque converter.

What does that mean?

In an automatic transmission there may not be a direct linkage with the engine, to the gearbox.
It might be depending on the design, connected through liquid.

One side spins the liquid(connected to the engine) and the other side gets spun around by the liquid.
This can be changed by moving the distance, but even when farther away, there is still a little bit of momentum transferred between the liquid.
Think like having two blenders with one upside down on the other, and powering the other one by turning on one.

There can be direct links which are better for gas mileage but change the dynamics of the car, but that’s getting a bit beyond the general overview.

Great answer, till you said breaks🤦.

Breaks on a vehicle are bad and should be repaired.

Brakes on a vehicle are necessary and should not break.

Oops my bad. I’m on my phone so I’ll blame that lol

Additionally at idle the brakes can overcome the force of the fan and stop the car. Without brakes the car will move under idle power

BRAKES

I’ll figure it out eventually 😬

Yes! This is the best answer here. It's known as "auto creep" or "idle creep" if anyone wants to read up about it.

same for mopeds and motorcycles etc. The engine and transmission are connected in gear(drive) when two are linked and it's a matter of whether enough horsepower is being generated to move based on current RPM, if not enough or on a hill or carrying load all require more engine torque to get moving initially

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