what exactly does ID mobile do?

If you’ve cancelled then I guess it no longer matters and I guess you meant what were you paying for. Regardless I’ll try to answer this.

They, being a mobile virtual network operator (MVNO), provide a mobile network service, though most of us tend to say a mobile phone service or mobile phone provider, whether that be a SIM, or eSIM, with or without a phone either bought outright or purchased throughout the duration of your 12/24 month contract.

They’re focusing on providing value packages with simple features.

They, being a Currys owned business, would have negotiated a deal with Three UK. That deal allowed iD to ‘rent’ part of Three’s infrastructure. Chances are the deal agreed allowed iD to offer their services at low cost, no frills. But yes as the old saying goes, you get what you pay for.

For me anyway, it was the cheapest provider for what I get - unlimited minutes and texts and (although I don’t need this) a higher data bundle than what I had with my previous MVNO. Going direct to Three was way more expensive. So for the price it was a very cheap way to get a SIM.

(I highlighted the main answer in bold)

Should you require more information, please feel free to read this wiki result - https://en.wikipedia.org/wiki/ID_Mobile

Now I am getting emails from solicitors asking for money regarding ID mobile 

I don't get it.

My phone doesn't work any differently since I cancelled or stopped paying so I am guessing I don't need it.

What should I do?