Connecting different Sensors / Actuators
Most microprocessors/microcontrollers have on-board bus systems like CAN, I2C, and SPI, OWI, RS485 (via USART/UART) etc that don't require a separate processor for bus control (although you may need a transceiver in the case of CAN or RS485 and some others). These are bus-specific protocols, and you would pick one (or more) that fit the types of devices you have on the bus.

The actual architecture depends on the type of bus you select. I2C has 2 wires (data and clock) and SPI has 4 wires (MOSI, MISO, CS, clock). Each bus has particular timing requirements, maximum lengths, maximum devices, addressing schemes, speeds, etc. Much of the bus protocol is abstracted away in the MCU/MPU interfaces and communication libraries, so you only have to worry about actual data transmission rather than what the wire is doing.