The iPads jack input/output can be used to record a signal but you’ll need to connect to the pad with a TRRS cable. This is how the mics that plug directly to your headphone socket work. You may also need a special cable to have a line level signal to work but I haven’t tried this myself. Check out this blog post.

I assume you are using some kind of usb mic with a headphone output? In this case I would instead connect the mic directly to the iPad with a camera connector kit or usb c if it’s a newer iPad. If you must use the headphone output, you will need a trs- trrs cable such as a Rode SC7 but this may not work as intended with the signal levels from the headphone out jack of your mic.

An iPad doesn’t have audio in typically. The jack on it is audio out.

If you have, say, a laptop with a dedicated microphone jack, you can use that in combination with headphones and use those as a microphone. A mic is just a speaker with inverted membranes.

But like I said, it’s important for the audio host to have an input port.