[apalrd]

Just to be clear:

UART is 'Universal Asynchronous Receiver Transmitter' and is a hardware peripheral on microcontrollers used to implement a bit-serial interface. It is essentially the hardware that serializes and deserializes bits from a serial bus into bytes, and handles clocking. It by itself has no voltage, that is dependent on the implementation.

TTL is 'Transistor-Transistor Logic'. Usually this is defined so 0-0.8v is logic 0 and 2.2-5v is logic 1, so 3.3v and 5v are both acceptable voltages although some 3.3v devices do not tolerate 5v. TTL is a digital signal spec, it does not define what the digital signal is.

RS-232 is a standard which uses +-12v as logic 1 and 0. It defines the signal voltages, timing, and connector. The timing can be generated by a UART.

RS-422, RS-485, LIN, and several other physical layer standards also use UART with a different physical transceiver. The signaling voltages are different, but the timing is the same.