The sensors could be nothing but simple conducting non-corrossive metal rodes.
The IC at the right hand can’t be 555, it is ULN2003. I don’t know which is the inverter IC used.
It works on the basis of setting and resetting the timer IC using the trigger and threshold pin.
Note that the Low probe is connected to trigger pin and High probe is connected to threshold pin.
Just like normal capacitor operation of timer circuits, when the water is at low level the timer IC triggers and output will be high, and water reaches High probe the timer get enough threshold to turn itself off. The SL100 is used to drive the relay based on the output from 555.
That’s how I read the circuit.
Keep the circuit as a reference and try to design your own!
If you want to know more about the 555 operation, search for 555 internals and working. You can find lot of tutorials.
One such link is given below.