You several times mention both UART and RS232 which makes me think that you are not clear about these terms. 'UART' normally refers to the hardware module used to generate and read the signals being sent and received. This implies the use of the asynchronous serial protocol of a start bit, a number of data bits (normally 8), perhaps a parity bit and between 1 and 2 stop bits.
RS232 is an electrical standard that defines what voltage a '0' has on the line, and what voltage a '1' has on the line. ('0' is typically between +3V and +15V, while a '1' is typically between -3V and -15V - obviously with reference to a common ground between the sending and receiving devices.) The protocol also defines other characteristics but that are not relevant at this point.
What I think you are considering is the voltages that most MCUs use which is either the TTL levels ('0' being 0V, '1' being 5V) or similar voltages for the serial signals. (0V and 3V3 are also common voltage levels).
Susan