LEDs are mostly current-controlled devices. But you do need to limit the voltage so that the current will not exceed its rated value. The common voltage for the red LED bead is 2 V, and 3 V for the blue ones. In a 12 V DC current, there are usually 3 LEDs and a resistance to control the current. As for high-power LEDs, the voltage can be between 90 V and 260 V. The transformer will adjust the voltage to the needed value. Through PWM technology, the current in the circuit is controlled.
More reading: LED PCB Design Guide: Challenges & Considerations