Hello Pranesh, the modern day automatic transmission can be classified in three types.
1). Torque converter type
2). AGS (Auto gear shift)
3). Variable Belt type
Going into the technical aspects of each type would be out of scope in this forum. Sufficient to say that each one has its strengths and weaknesses and have been correctly mated to the engine by each manufacturer.
Just remember its like having one more servant in the home. More convenience but you have to pay more initially.
Also even in a best case scenario, the manual transmission will get at least 2 kms more mileage than the auto version.