This is not about "external noise". The USB wires are differentially coupled TWISTED PAIRs, so you will be badly affected by signal coupling if the cable is long.

Yes, a RS232 to USB converter does have a full-blown UART circuit built-in. As any standard UART, it does have Rx, Tx, and flow control pins. As any UART, it must be properly configured to be able to communicate with the "partner" at the other end of UART link.

To do so, the UART circuit is "bridged" to USB interface. The USB interface has the CDC class descriptor (Communication Device Class), and the USB host driver maps this USB device into a virtual COMnn port with nearest available number, so PC applications can use it as an usual COM port.

The bridge is fairly simple. To read the UART data, the USB COM port driver is constantly asking the USB device for data (since it has no other means to know when the UART will receive its data and will be ready to give them to host). When there is no data, the bridge responds with NAK. When data is there, the USB side returns the piece of data.

When the host wants to send a UART data, it issues an OUT packet with 1 byte of data. The bridge receives the serial byte into a parallel buffer (at USB speed), and then stuffs the received parallel byte into the parallel side of UART for transmitting it at UART's speed.

Before communicating over UART, the USB side must configure the UART for proper baud rate, stop signal length, and use of flow control, as in case of usual UART in a PC. This is virtually transparent to application terminals, and the terminals use the usual interface to set these parameters up.