I cannot see the difference between the "imperfect tense" and "avoir l'habitude"

Bonjour Stephen ! The difference is one of emphasis. L'Imparfait can carry the notion of something that *used to* happen, as opposed to Le Passé Composé, but "avoir l'habitude de" (literally "to have the habit of") insists on the notion of a repeated action in the past, something that happened as a habit. In the example you used, saying "J'avais l'habitude d'appartenir" sounds really weird, because "avoir l'habitude" you wouldn't really "belong" repeatedly, as a habit :) Here you will use L'Imparfait : J'appartenais, as it's more of an action that used to *be* in the past. I hope that's helpful! À bientôt !