I used to think that Canada had a lot of French immigrants, but I've come to realize that Canada has an abundance of immigrants from other countries as well.
ご質問ありがとうございます。
「人口構性」は英語で「population structure」と言います。
また、「カナダはフランス[移民](https://eikaiwa.dmm.com/uknow/questions/3523/)が多いと思ってましたが、他の国の移民も今はものすごく多いそうです」と説明したいなら、英語で「I used to think that Canada had a lot of French immigrants, but I've come to realize that Canada has an abundance of immigrants from other countries as well.」になります。
ご参考になれば幸いです。