Nuuu, apparently you are not such an ectomorph. My weight is 45kg, height is 170cm. I worked in the gym for 6 months, and gained only 3 kilos. As soon as I quit classes and heavy meals, the weight instantly returned to 45 kg. My husband is also thin, 64kg and 185cm. In 2 years of training, he was able to achieve only 68 kg, and on this growth stood up.
We have been like this since childhood, and no matter how we exercise / eat, the weight grows very hard.
Muscle fiber, whatever it may be, grows from a certain load. Or do you really think that ectomorphs have some special type of muscle fibers, not known to science, who knows how to train?
I always give this example. Imagine an expensive foreign car and our Zhiguli. Your task is to get from point A to point B. The difference in the power of the cars will allow one of them to get there faster. But does this mean that the second will not get there? No. It just takes longer. The same story with ectomorphs.
It is more difficult for them to gain muscle mass, not because their muscle fibers develop in some special way, but because they simply initially, genetically, have much less muscle mass. Accordingly, in order to gain the mass that the mesomorph initially has, they just need to spend a little more time on it.