What do I dispose a completely smashed iPhone?

Do an internet search to see if there is local electronics recycling in your area.

Or go to Apple Trade In - Apple

and scroll down to "More ways to recycle" near the bottom of the page, click that, then click the "click here" in that section.

joziie ~ Apple makes it easy to recycle your device in your country or region:

Apple Recycling Programs

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If it still works at all, you can attempt to sell it to an ecoATM machine - I see that it says an iPhone X. Is this correct? If so, you can go on their website and click "iPhone X - broken - get offer" and see what they have to say.

In that case, do what Alancito said and recycle through Apple trade-in.