What are the main reasons of condom breaking? and if it breaks, it is obvious or it can be just a small fissure ?

Applied wrong: If there is not a space left at the top for the ejaculate (if it does not have a "reservoir tip"), then ejaculation can cause it to rupture. It is easily noticed when it fails.

Both, possibly: Most condom ruptures are due to excessive friction, due to lack of adequate lubrication, but can also be due to using the wrong lubricant. You need to use water based lubricant, not petroleum based lubricant, like vaseline. Vaseline will break down latex condoms. As to how a break occurs, they generally tear themselves apart, but could have a smaller leak possibly.