The triangle is an opamp. Its output (right) is the difference of inputs (left) (plus some small offset due to manufacturing tolerances) multiplied by a large number (million or more), capped to more or less the voltage of its power rails. In most uses, there is a feedback so the output influences inputs to equalize them, effectively creating an amplifier whose voltage amplification is set by rhe strength of the feedback. Without a feedback, the opamp’s output will be either GND or Vcc depending on the which input’s voltage is higher. An opamp in this configuration os known as the comparator. Basically, it flips based on whether the photodiode is generating voltage, and I think that allows for higher frequencies than a traditional phototransistor. In reality, a very large resistor (megaohms or gigaohms) is added so that the voltage on the diode actually goes below zero when no light is incoming but it was omitted from the diagram.

The rectangle looks a bit like a European-style potentiometer but it isn’t. Maybe a resistive divider?

Maybe two outputs because DIP-6 packages are uncommon and they went for the standard.