Waiting for a loop containing asynchronous calls to finish

I have been trying to find a solution to a problem regarding an asynchronous method, I have tried various types of loops, using async/await, as well as Promisies, but none I try seem to work for me.

So I have my first method:

const postApplication = async (params...): Promise<DocumentReference> => {   const formData = await prepareData(someParams...)   {rest of method...}}

I then have my prepareData function, which I believe is where my issue:

P.S. I am only keeping the information that I think is necessary to make it simpler.

const prepareData = async (params...) => {    const formData = new FormData();    {some synchronous code...}    for await (const field of allCustomFields) {      if (field.identifier === someProperty) {        formData.append("fieldName", field)      } else {        for await (const value of field.value) {           const storage = getStorage();          if (value) {            const fileInfo = value as FileValue;            const imageName = fileInfo.firebaseStoragePath              ?.split("mutable/")              .pop();            getDownloadURL(              ref(storage, `users/${user?.id}/mutable/${imageName}`)            ).then((url) => {              const xhr = new XMLHttpRequest();              xhr.responseType = "blob";              xhr.onload = (event) => {                const file = xhr.response;                const extension = file.type.split("image/").pop();                formData.append("filesToUpload",                  file,                  `${field.identifier}-${docCounter}.${extension}`                );                docCounter += 1;              };              xhr.open("GET", url);              xhr.send();            });          }        }      }    }   return formData;}

So the problem I am having, is that formData is being returned before the files I am retrieving from Firestore have been appended to formData. I have tried each of the loops in a Promise and then calling await Promise.all(nameOfConst) before returning formData, but none seem to work.

I appreciate anybody that can offer some help,

Thanks