RS232, if I'm not mistaken, is a physical standard, not a protocol. That is voltage levels and connectors, and the timing requirements of the other lines, CTS, DTE etc... The format of the data actually sent along an RS232 channel isn't part of RS232, but it almost always a typical asynchronous UART format. That is start bit, data, (parity) stop bit. With typical baud's of 2400 - 115200 bits/s.
So in answer to your question. They are 'used in conjunction with one another'.
I2C, as nigel perfectly explained is just to reduce the need for lots of PCB traces between communicating IC's, its addressable so multiple devices can be connected to the master with the same two wires. SPI is similar in that its synchronous (has a clock line) but there is no adressing, just a 'chip select' line for each device..can go up to 20Mhz. U(S)ART comms is generally async, two lines can give full duplex comms at a fairly slow bitrate, but it is simple and easy to setup so its still used today.
Blueeeth