How does algae always grow?

It was probably due to the interparticulate interchange between the air and water surface that algae get up in the air.

Quote from Lucas

I see, what makes them fastest growing ?

Not sure, but they can be spread by rain and dust particles, likely they were in a cyst state during this process.

Quote from Lucas

Mostly they speared everywhere, is there any place where they don't grow ?

They can't grow in a sterile environment unless introduced. Once there are enough nutrients available for growth they will grow

Quote from Lucas

It can be problematic for aquariums right ?

Yeh, it can be potential problem for aquariums, can be quite a beneficial source of nutrients for fish in your tank

Algae can be carried in the air as well. This has been known forever.

Quote from Lucas

You can put tap water in a jug, seal it up, leave it in the sun for two weeks and lo and behold, algae will still grow. Where does it come from?

It's good that you mentioned sunlight, it's one component that helps algae to grow out of no where but there must be another variable available as well to combine with the sunlight before they can grow and it's the availability of nitrite and phosphate in the water.

You probably notice that algae once it starts growing will just sprout and continue to grow rapidly. More so in pools and also in aquariums when they are in the sunlight you will notice it the most. Algae can also grow using other forms of energy such as sugar and even carbon dioxide. It actually is really amazing what you will find helps algae grow and how much you did not know feeds it. I was amazed myself.