cmos as amplifier
You are right. The CMOS inverter mostly is used in digital circuits.
However, it has a rather steep transition range between the on and the off state.
Always, such a region can be used for amplification (like in each "normal" amplification stage), when you can fix a bias point in the middle of this range.
And, luckily, this is possible with a CMOS stage. This is because an input voltage of Vdd/2 (at both gate nodes) will produce an output voltage at Vdd/2 (middle of the transition region). Therefore, connect a rather large resistor between output and input - and you have a fixed bias point due to negative feedback. If you then inject a signal via a coupling C you have an inverting amplifier. As an alternative, you can inject the input via another resistor (lower than the feedback resistor) - and you have an amplifier with signal feedback and lower gain. Try it.
Regards