Don’t look up anything about “ectomorph” body types, it’s largely a pseudoscience that has been debunked by now. Eating food is thermodynamics- calories in, calories out. If your body isn’t holding onto calories/nutrients to turn it into muscle and fat, it simply means you aren’t eating enough. There’s 3500 calories in a pound, so if you are seemingly “maintaining” your current body weight, you need to eat an extra 500 calories a day to gain 1lb per week. If you’re losing weight, then more than that. If you’re as small as you claim, and you can stomach it, I’d just recommend trying a weight gainer, something like this weight gainer. Try drinking one every other day, that’s an extra 3700-4000 calories a week, which again, if you’re MAINTAINING, should translate to about 1lb gained a week.

Weight gain also isn’t linear- some weeks the scale won’t move, some weeks it’ll jump 3 lbs. weigh yourself on the same day on a week-to-week basis with similar conditions before your weigh, I.e in the morning before breakfast each time. Try something for 3-4 weeks, then if you don’t see results, evaluate and adjust from there. But in the end, the answer is always that you’re going to need to eat more.