Where's the Polish ą on my iPhone 8 ?

Under the regular "a" key if you have the Polish keyboard set as an available keyboard option and it is the currently selected keyboard. Press and hold the "a" key for extra character options.

Go to Settings->General->Keyboard->Keyboards to add other language keyboards.

ski3 wrote:

Of course this begs the question: Why was every Polish letter using diacritical marks except the nasal ‘a’ available without bringing in an additional alphabet?

Good question.

BTW, That little globe icon in the bottom left corner is how you switch between keyboards.

Yes.

You set the language prior to using the device. It's one of the first things you do when you set up the device.

Works for me: