这句话的主干是 this is where A comes in.
A = developing new habits 动名词短语做主语,意为养成新习惯。
come in 这里的意思是起作用,发挥作用。用英文解释就是If something or someone comes in, you are talking about their role in a particular matter.
翻译过来就是:养成新习惯就是这个地方起作用的。更清楚的表达就是你的答案中提到的。
不必为此纠结,把这句话当做固定表达法记住就好了。
追问谢谢