I have built a custom hook:

const useFirestoreCollection = async (collectionName) => {

    const [documents, setDocuments] = useState([]);

    useEffect(() => {
        const fetchData = async () => {
            const querySnapshot = await getDocs(collection(db, collectionName));
            const documentsData = [];
            querySnapshot.forEach((doc) => {
                documentsData.push({
                    id: doc.id,
                    ...doc.data(),
                });
            });
            setDocuments(documentsData);
        };
        fetchData();
    }, []);

    return documents;
};

I can use it in my component like this:

const [myData, setMyData] = useState([]);

useFirestoreCollection('registeredUsers')
    .then((data) => {
        setMyData(data); // Data Access
    })
    .catch((error) => {
        console.log(error); // Error
    });

I can also remove the async in the custom hook:

const useFirestoreCollection = (collectionName) => { <- ASYNC DELETED
    const [documents, setDocuments] = useState([]);

    useEffect(() => {
        const fetchData = async () => {
            const querySnapshot = await getDocs(collection(db, collectionName));
            const documentsData = [];
            querySnapshot.forEach((doc) => {
                documentsData.push({
                    id: doc.id,
                    ...doc.data(),
                });
            });
            setDocuments(documentsData);
        };
        fetchData();
    }, []);
    return documents;
};

...and use it in my component like this:

const myData = useFirestoreCollection('registeredUsers')

I tried to figure out the best way but couldn't.

When should you use the async keyword in hooks, and when shouldn't you use it?