When the circulating supply increases, we can say "new bitcoins were mined"?
To have it clear, new bitcoins are created or we "discover/found" it?
in a simple explanation "Mining" stay for use computational power to resolve a mathematical problem.
At the beginning this problem had a low complexivity, and could be solved with a low computational power.. for example in a "SOLO MINING" mode...
by the difficulty increase, this problems become ever and ever more complex... and to solve this problem was born the so called "MINIG POOL".
When a miner, in solo or pool mode, mine a block, means who have found the solutuions to this problem and it is rewarded by 12,5btc + commission fee