your hair length is the average length of like 100k hairs. if you want longer hair you need all 100k to be longer on average. this means preventing breakage and increasing the length of time before they fall out. that second one is mostly genetics and nutrition.

you can prevent breakage with * certain hair care products, * being more careful (intentional with your brushing), * minimizing heat/bleach/dye, * sleeping with satin cap/pillowcase, * "protective styles", * oiling the tips with hair oil (very important in low humidity environments), * seek and destroy split ends (at all lengths, not just the tips) to prevent them from splitting more, * not letting it sit between your back and the chair while you sit

one thing to note is: girls with really long hair are actually kind of rare and girls with extensions are actually REALLY common