Regarding bitcoin circulation, around 19 million bitcoins have already been mined, leaving less than 2 million bitcoins to be mined. After every 210,000 blocks mined (roughly every four years), the block reward given to Bitcoin miners for processing transactions will be cut in half.
Two primary sources of income for bitcoin miners are block rewards and transaction/gas fees. In the long run, once bitcoins are all mined, transaction fees will be more expensive than now to incentivize miners to secure the bitcoin network since there will be no more block rewards.
One thing we can conclude now is that bitcoin won’t be used for micro / nano payments since fees will be super expensive.