> There birthrate of Japan is considered low, but they are growing still

I don't know where you are getting your information that the population of Japan is still growing. Japan's population is already declining.

http://www.stat.go.jp/english/data/handbook/pdf/2017all.pdf "Statistical Handbook of Japan" Table 2.2

https://www.populationpyramid.net/japan/2020/ ( https://www.populationpyramid.net/sources )

https://www.worldometers.info/demographics/japan-demographic...

https://qz.com/1295721/the-japanese-population-is-shrinking-...

https://www.nippon.com/en/japan-data/h00705/ "Japan’s Population Falls for Ninth Straight Year"

https://www.ft.com/content/29d594fa-5cf2-11e9-9dde-7aedca0a0... "Japan’s population decline accelerates despite record immigration"