impedance matching between opamp and microcontroller (or Arduino)
You don't always need impedance matching.

Generally it is only needed in linear systems where the inputs and outputs have a characteristic impedance.

This is usually only the case for RF (high frequency) signals and other cases where the length of a cable (or transmission line) is much longer than the wavelength of the signal.

Examples:


The connection between an RF Power Amplifier (PA) to its antenna, this might work at 900 MHz, that's RF. You want all the power from the PA to arrive at the antenna so you must impedance match or power will be lost which is wasteful.
The old landline telephone system. These have a 600 ohms characteristic impedance. Why? Because of the cables, these have a 600 ohms characteristic impedance so we drive the cable from a 600 ohm impedance and we also need to load it with 600 ohms. This prevents signal reflections and distortions. Land phone lines can be long so this is an issue even for low frequency signals like speech.


These above examples are both transmission lines.

Read that Wikipedia article and note how both ends of the T-line need to be terminated properly. That's the impedance matching.

Opamps do have a certain output impedance, like 10 ohms, but that is not a characteristic impedance. Most opamps are not designed to drive a similar value impedance. And it is not needed as we consider an opamp's output to be a voltage source. Also, we usually do not use an opamp to drive a transmission line nor do we use it at RF frequencies.

So no need for impedance matching with an opamp!

Unless you'd use a very long cable and want to use that as a transmission line and avoid signal reflections.

Similarly microControllers have high impedance inputs, these are not designed to take a signal from a transmission line. And we don't need that. The opamp will often be just a few centimeters away from the microController's input so no transmission line is needed.

Also, the information in the signal coming out of the opamp is a voltage, not a power, as long as that voltage arrives at the uC's input safely that's enough. There does not need to be any power matching.

Actually in the opamp to uC input case you have maximum mismatch but that's OK. The opamp has a low output impedance, the uC's input has a high input impedance. This is the best way to transfer a voltage while not wasting any power.