What happens to IOTA after use, do they expire or are they recycled somehow and does this reduce the available supply or is it somehow replenished ?
They don't expire. It's just like a bank account. We have a total amount of IOTAs (2,779,530,283,277,761). This amount is distributed through many wallets. You can look up the distribution here.
It's the distribution of a snapshot from August. I think it has changed dramatically in this few last months. The IOTA Foundation will probably release a new distribution overview later.
You can buy some IOTAs on an exchange. On the exchange these IOTAs are only virtual, until you transfer them to your private wallet. The exchange provider will then take the coins from the available wallet (other people/traders like you) and transfers it, through the tangle, into your wallet. The total amount will not reduced. It will just transfer from one wallet (another person) to your wallet. It's like with your bank account. You transfer your money to another bank account. There is no expiring of your fiat currency. Besides the inflation-rate, but that's another topic.
Trade in MiOTA not IOTA
It's important to understand that we are currently trading in MiIOTA. So, you get one million iota, not only one.