Heyo 
– some really good questions here.
Obviously, there’s nothing explicitly built into Directus for managing inventory, so you’d be building solutions for these yourself.
I would definitely be looking at custom event hooks through extensions. Here’s some pseudo code of what it might look like for something like that:
// You can leverage the database instance, which is just the
// Knex instance that we use within our own services
// extensions/hooks/inventory-management/index.js
export default ({ filter, action }) => {
// Use filter to intercept BEFORE the item is created
filter('items.create', async (payload, meta, context) => {
if (meta.collection === 'order_items') {
const { database, schema, accountability } = context;
await database.transaction(async (trx) => {
// Lock the product row for update
const product = await trx('products')
.where('id', payload.product_id)
.forUpdate()
.first();
if (!product) {
throw new Error('Product not found');
}
if (product.stock_quantity < payload.quantity) {
throw new Error(`Insufficient stock. Available: ${product.stock_quantity}, Requested: ${payload.quantity}`);
}
// Update stock atomically within the same transaction
await trx('products')
.where('id', payload.product_id)
.decrement('stock_quantity', payload.quantity);
});
}
return payload; // Important: return the payload for filter hooks
});
The normal behavior is “whoever saves first wins” approach, but using database transactions should get you closer to what you’re after or throw an error if there’s insufficient inventory.
The other side of the coin is, of course, you could build just about anything with Directus, but that doesn’t mean that’s always the best course of action.
If you’re building a really complicated e-commerce system where you’re managing inventory of hundreds or thousands of products, it might be best to use an already existing dedicated e-commerce solution rather than having to rewrite (and maintain 
) all of that logic yourself.
Something like Commerce Layer or Medusa, for example. You can leverage Directus for product management and website content. And then use headless ecommerce systems to manage all the inventory and transactional lgoic.
Hopefully that’s helpful! 