How can a DC-DC converter output regulated voltage?

I found some DC-DC converter has a fixed output voltage, that is, when input is 2.7v-6.0v, output voltage could be regulated at 1.8v.

How to achieve it? Is its efficiency as high as normal DC-DC converter?

Well the mechanism of regulation is through the duty cicle = ton/T. That means the regulator only let pass th energy needed to obtain the 1.8V.
Efficiency is obtained by the losses in the devices and the conmutation, if ton is very small the eficiency become small. That is because the losses in switching are very similar to ton.
The rule is Losses by switching must be less than 10 times the energy in ton. So losse can be negible.

For any DC-DC converter, there is a feedback loop to sense the output voltage and apply either more or less input voltage in order to regulate to the desired output. Usually just an op-amp comparing an internal reference voltage (bandgap voltage) to a feedback pin (FB).

A linear regulator uses a pass device operating in the linear region - it allows more current to flow if the output is getting low, and restricts the flow if the output is getting high.

A PWM switching converter applies pulses of VIN to an LC filter. If the output is getting low, the pulses increase in width to give more current to the output. If the output is getting high, the pulses decrease in width. The width of the pulses is determined by the error amplifier watching the output through the FB (feedback) line.

For example, if the input is 2.7 and the output is 1.8, a PWM converter would be giving pulses of about 66.7% duty cycle, since 1.8v is 66.7% of 2.7v. If the input voltage was 6v, the pulse width would decrease to 30% duty cycle to keep the output at 1.8v

Is it clear?