Isolation is commonly used in modern electrical systems, such as industrial automation systems, motor drivers, medical equipment, solar inverters and power supplies. Sometimes it may concern the designer whether the two sides of a digital isolator can work under different voltages in their system design.
Digital isolators can work under the voltages within the recommended operating conditions on both sides of the device. Since there is an isolation barrier between two sides, each side can work independently under the recommended operating conditions. For example, ISO7721 from TI can work with 3.3V for Vcc1 and 5V for Vcc2 (ISO7721 supply range is 2.25V to 5.5V). Then you can use the digital isolator as a logic-level translator in addition to providing isolation. The two sides of the isolator are independent as in the below typical ISO7721 circuit.