What happens when the iPhone warranty expires? ?

Repairs are free when you have a warranty.

You can purchase the extended applecare warranty for $250 but first see if it covers that particular iPhone 

That's poor people's question. Just get a new phone.

Same as any appliance - the item is no longer covered by the guarantee.

What "happens"? Nothing "happens". What un-happens is that the warranty is no longer in force.

It explodes as a very small mushroom cloud

Check if T-mobile offers an after market warranty but it's not going to be cheap.

you need to call the phone company and ask them this

You're on your own and will be responsible for any repairs needed.

Then, any repairs that phone might need will be paid for by you.