In CloudFirestore how do you add URl in the field section in Document?
As suggested by Vinamra Jaiswal, you need to store the URL as a string in the Firestore, you can refer to "How to store multiple url in firestore using flutter" for an example.
As for the image you can refer to "firestore database add image to record", where you can see that it is wiser to store the images in the Cloud Storage and then download the URLs and store them in Firestore.
You can also refer to the "Upload image to Cloud Storage, save URI in Firestore" video to get more insight on how to do this.