It’s as complicated or fancy as you make it. I like doing this stuff for fun, so fancy.
I have a saltwater pool, except for salt, hardly needs any chemicals.
The saltwater chlorinator is controlled using RS485 via USB adaptor
2 relays reserved for acid, and Clearwater chemical injection
1 relay for pump on/off
4 relays for primary pump variable speed control
1 relays to switch between primary and secondary pump
1 relay for secondary pump hi/low speed
2 relays for water flow pool or spa (actuators)
1 relay for solar heating (actuator)
1 relay for pool vacuum (actuator)
1 relay for spa blower
1 relay for pool & spa lights
1 relay for garden lights
Also got an analog pressure sensor to stop the system on low or high pump pressure events, and an analog day light sensor to automate the pool/garden lights.
Several 1wire temperature sensors for the solar heating logic.
Since the world of pool equipment lacks some serious standardization, I build a system that can deal with equipment of many different vendors. This also meant that I needed to have DC: 3.3, 5, 12, 24 volt and AC: 24 volt power available to select from. I included a 24AC transformer to get the 24 VAC and 24 VDC, and a 12 VDC psu from which I also create 5 and 3.3V DC using bucks.