Firestore - How to retrieve all subcollections of a collection

Question

I have a question regarding Firestore, I created a "category" collection in which I have several documents, I created a "construction sites" sub-collection! I want to retrieve all my sub collections from each category, but when I try to do like this:

useEffect(() => {
    const listConstruction = [];

    db.collection('category').get().then((collectionCategory) => {
      collectionCategory.forEach((doc) => {
        doc.ref.collection('construction').get().then((collectionConstruction) => {
          collectionConstruction.forEach((doc1) => {
            listConstruction.push({
              idCategory: doc.id,
              libelleCategory: doc.data().libelle,
              title: doc1.data().title,
              description: doc1.data().description,
            });
          });
        });
      });
    }).finally(() => {
      setConstruction(listConstruction);
    });
 }, []);

The problem is that it hasn't finished pushing the constructs it passes into my finally!

Having no experience with NoSQL and Firestore yet, I would have liked if I'm not completely wrong about its use and if so how to fix it?