Turn on inventory tracking per SKU or variant, and BigCommerce decrements the count automatically as orders come in. A low-stock threshold triggers an email notification to the store admin, and a separate setting controls whether the product keeps selling, shows as out of stock but still orderable, or gets hidden entirely once inventory hits zero.
Bulk stock updates run through CSV import and export, which covers most day-to-day adjustments, a price change, a restock count, without needing to edit products one at a time in the admin. There's no native purchase-order feature tracking what's been ordered from a supplier or when it's expected to arrive; a restock is just a manual number change whenever someone updates it.