How does d1 and d2 and d3, d4 gears work in an automatic car?

D1, D2, D3, and D4 work the same as Drive, except the number is the highest gear the transmission will shift into for each respective setting. D1 will not shift out of first gear, D2 will shift from first to second only, and so-on. These usually exist for towing situations, but there may be other reasons for them as well.

You wont need the lower ones for a normal drive. Only needed for steep hills when you want some power without the gear shifting and steep downhill when you want to control the gear shifts. Some other uses may be for tow.

D4 will automatically shift the gears from 1 to 4 for all normal driving conditions.

They don’t. It’s a fantasy.

Put it in reverse and go forward

Alabamian style