Create Stock Quantity Variable in Firestore with Flutter

I want to create a collection in Firestore through flutter where I can store a quantity variable which gets updated when I stock in and stock out items through my flutter app. However I can't seem to understand how to create that. How should I store that and how should I update the quantity field on stocking in and out items?

A collection is automatically created when its first document is created.

So if you need to manage only one stock you can create (initiate) a unique document in e.g. a stock collection and assign the initial stock. This initialization can very well be done through the Firestore console,

Then, to adapt the stock quantity, it depends on your global functional requirement:

1. If you just want to increment/decrement the stock quantity without impacting any other document in Firestore you can use the FieldValue.increment() method (use a negative value to decrement).
2. If your operation must be transactional and involves some other Firestore document (e.g. you modify a bank account, or a e-commerce basket, or another stock in parallel to this stock operation) you should use a Transaction.

You may also consider if you really want to allow this operation from your Flutter app and not from a back-end (e.g. Cloud Functions). To allow end users to do this from an app you'll have to open the write access to one or more collections and a malicious user could change the stock value in an undesired way if you cannot implement a security rule that prevents that.