This "equivalent input circuit" shows a resistor and capacitor at the input of optoisolator (inside the red oval):
Are they used just for modelling, or there are actual physical components used in this circuit?
Adding the source as requested. This is a diagram from datasheet of P1-16ND3 by AutomationDirect: https://cdn.automationdirect.com/static/specs/P1-16ND3.pdf.
These resistors and capacitor are probably physical, real components installed inside the module.
The left-most input resistor is clearly for current limiting, which is why you can connect 12V or 24V directly to the input. You wouldn't be able to do that directly across the LED without destroying it.
The right-most resistor is no doubt to prevent small currents from illuminating the LEDs. LEDs tend to glow quite brightly with only microamps of current, and that resistor provides a path for it, rather than through the LEDs. Such current can be due to electrical noise, or magnetic induction, and module this appears to be for use in an industrial setting, where there's plenty of sources of those.
The capacitor is probably another measure to mitigate noise picked up from nearby sources. In combination with the left resistor it slows the rise and fall of LED current, by forming a low-pass filter.
The resistor to the right of "INPUT" both limits the current through the LEDs and is part of a voltage divider.
The red-circle capacitor is a noise filter, working in conjunction with the input resistor.
The red-circle resistor both attenuates the input voltage and assures a rapid turn-off of the LEDs when the input voltage source goes open-circuit, such as disconnecting the input signal with a switch.
Note: The symbol for the optical isolator is incorrect. The two LEDs should be shown separately. What the drawing has is the symbol for a DIAC, which probably is incorrect.
Please update your question with the part number and web link for the module.
It says on the datasheet that it's an equivalent circuit, not actual.
The actual circuit can be more complex but for purposes of anyone using this component, that level should be enough.
And that model tells that as the optoisolator input is likely a LED with about 1.6V voltage, the input series resistor is to limit current when voltage is 12V to 24V, and the pulldown resistor is to keep stray voltages from turning it on, as well as the capacitance to slow down signals and ESD for filtering effect.
There could be protection as well.
