I think what you are asking is about just learning basic electronics and the digital protocols that connect different "modules" (and what kind of extra circuitry they need). For most consumer stuff (i.e. if you are not working on cars or industrial robots) you can get a lot done with just knowing about these three protocols:

- UART: https://en.wikipedia.org/wiki/Uart

- SPI: https://en.wikipedia.org/wiki/Serial_Peripheral_Interface_Bu...

- I2C (sometimes called TWI - two wire interface): https://en.wikipedia.org/wiki/I%C2%B2C

For more links for learning electronics check out my list of resources: https://github.com/monostable/awesome-electronics.