What does it take to get a BMI of 18?

I've never been very heavy, the heaviest I've gotten is about 23 in BMI. However, at the same time I have never dropped below 20.5 regardless of how hard I tried. I previously went to the extremes of weight loss, dieting so to eat 700 calories AVG. a day (I tracked everything I ate, weighted it etc.) and exercised about 30mins every other day (usually running outside or gym). Was doing yoga and tai chi classes and walking 10k/15k steps a day on top of that but my weight loss stagnated at 56kg. I had to abandon this because it was interfering with my life and studying.

6 months and my weight was back to 67kg. Once I was done with studies I started re-attempting my goal of 52kg. This time I am keeping calorie intake to 1.2k so to not feel so weak and lightheaded but my exercise went up to 40mins vicarious exercise every other day (approx 4 days of cardio and 1 day of weights or 3:2 ratio) but my weight loss stagnated at 57.5kg for a month and hasn't budged.

For anyone who intentionally managed to achieve a BMI for around 18, how much work did that take you? Am I doing too little or being impatient? What do you think the reason for my plateau could be, too high calorie intake or not enough vigorous exercise, not enough moving around during the day? Just trying to understand what I'm doing wrong.

What does it take to get a BMI of 18?
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