You’re actually in a really good position to build muscle, being 18 and presumably a little underweight. (How tall are you?) Your body can grow like a sponge if you give it consistent training and plenty of calories.
You definitely don’t need to train 9 times a week like Cooper did. Any well-designed plan lifting 3-5 days a week will be effective. There are plenty to choose from. This is one good plan.
Stick to the nutrition principles laid out here - animal protein, a good carb source, and healthy fats in every meal. 3 meals a day, 7 days a week, a whole bunch of weeks in a row.
If you “can’t” gain weight, shoot for at least 6 whole eggs every single day. Doesn’t matter how they’re cooked (as long as they are cooked). Hard-boiled, scrambled. One egg six times a day or a monster 6-egg omelet, whatever. Old school lifters used to suggest drinking a gallon of milk a day, but if you can do six eggs a day instead, start there. And that’s in addition to your three meals, not instead of them.
As far as supplements, like SBT said, Plazma is hands-down top of the field to recover from workouts, but Surge Recovery is simple and straightforward, still plenty effective delivering quality protein and carbs that your body needs to rebuild after training, and (more importantly) much more likely to be in your budget. Add in a scoop of creatine and you’re set.
If you want to supplement your whole food meals with protein shakes, you can make your own “weight gainer” by using Metabolic Drive mixed with milk and adding stuff like peanut butter or fruit to it. So you’re getting good nutrition, protein, and calories instead of sugar-loaded pre-mixed weight gainers off the store shelf.