For some reason I thought "en" was used when referring to ideas or non-descript things?

Bonjour Laura,

Note first that "en" is never used to replace people.
As for things, it depends on whether the original phrase it refers to is introduced by the preposition "de":
E.g. Tu veux de l'eau ?     Tu en veux ?
E.g. Je me moque de ton pull.     Je m'en moque.

Here are links to the lessons explaining when to use "en":

https://french.kwiziq.com/revision/grammar/the-adverbial-pronoun-en-can-replace-a-phrase-introduced-by-de

https://french.kwiziq.com/revision/grammar/the-adverbial-pronoun-en-means-of-them-with-quantities

I hope that's helpful!

A bientôt !

 

Bonjour Laura, En is used to replace de + anything, whether concrete or abstract. When there's no de, there can't be en. Le is used instead.

"en" not used with people? How about this : Combien de personnes avez-vous sous votre commandement? J'en ai 100.

It is because it deals with quantity - As-tu un livre de français? Oui, j'en ai 100 - ( object) AS-tu des amis chez toi en ce moment? Oui j'en ai - (personnes) Combien d'employés avez-vous dans votre entreprise? J'en ai 10 // j'en ai 2000. ( personnes)