Just have a loop check if there is data to send (and UART ready to send one more character) or if UART has data to put into the buffer.

Or use an interrupt service routine (ISR) that gets trigged whenever the UART is ready for more characters, or have received a charcter. Get characters to send from output buffer. Put received characters in input buffer. Let main loop fill output buffer, or process data from input buffer.

There are no magic involved. It is a very minor step from send/received of a single character into send/receive of full text lines.