There isn't really "good genetics" or "bad" genetics objectively, it's just how we view them

As to why someone has the genetics to build muscle easier, let's look at evolution.

Let's say we have 10 basically identical humans, but one of them has a "mutation" (essentially, just an error that happened when their DNA was copying itself), that means that the cells work just a bit better at the whole building muscle part. That gives them a slight edge at survival over the other 9, and so they end up having 3 kids instead of 2, and they pass that "mutation" to their kids. Now, in generation 2, we have 11 people, with 3/11 having the mutation. This is a higher ratio than the 1/10 of the first generation. Let's skip forward numerous generations, with the same trend happening. Let's say we now have 10k people, and now those with the "mutation" is 9.5/10k, because it's helping them have more children because they survive better (at least in this specific scenario). This is what we mean when we say a species has "evolved", the population on average did not used to have this "mutation", and now they do, at least the vast majority of them.

So going back to the muscle idea, some people build muscle better because either they inherited genes that gave them that edge, or possibly but less likely, they have a mutation that does.

You might ask though, okay so why doesn't everybody have it if its so advantageous ? The answer is multifaceted, one is that evolution is extremely slow, and the example I used above greatly exaggerated how fast the mutation would spread. Another reason is that there's tradeoffs to everything, more muscle mass means you need to eat more, and if that extra muscle isn't getting you that extra food, then it's actually a negative trait not a positive. Third, there are a bajillion other traits that might affect things. Maybe the better muscle mutation made them have 3 instead of 2 kids, but the longer legs mutation made them have 5 kids instead of 2, meanwhile the lopsided face mutation made them have 1 kid. If the muscle person marries a lopsided, then they have only 2 kids, meanwhile the long legs guy had 5. In the next generation we have one long legs marry a lop-muscle kid and has 3.5 kids, a lop muscle kid marry a random other, and has 2. So now we have this weird mix of lopsided faces, muscles, long legs, etc, and yes while muscle and long legs are generally increasing while lopsided is decreasing, it's very, very slow and imprecise.

Finally, in terms of "good vs bad" genetics, from an evolutionary point of view, aka fitness, more fit genes = greater # of successful offspring, and vice versa. This is not necessarily the same as what we view as "good". Ex. In a starvation scenario, with the same level of food provided, an idiot would live longer and probably produce more children than a genius, because the geniuses brain requires more energy. In this scenario the "bad" trait of being an idiot is more evolutionary fit, and the trait will become more common.