Yeah, but the whole debate is about how much is genetics responsible for intelligence vs the environment. There was a claim that genetics is responsible for ~70

Yeah, but the whole debate is about how much is genetics responsible for intelligence vs the environment. There was a claim that genetics is responsible for ~70-80% of variance. Pick any intellectual ability and lack of environment can cause for an individual to not be able to achieve that, so how can genetics be responsible for that much variance? The question is where and how does the 70-80% come? How is it meaningful and how is it measured?