You shouldn't actually worried about what would happen when all bitcoins are mined.
You won't be alive at that time most probably as that shouldn't happen before the 22nd century.
As all coins would be mined, miners earnings will depend solely on the transaction fees.
I can't tell how much fees would be at that time because theoretically if things go as we want, bitcoin will be the main currency used by humans so fees won't be compared to USD.
Where they put the transactions? They have to find a new block for this as far as I know.
Still in blocks in the blockchain of course but there won't be rewards for finding blocks.