How does the Shop app know what to track?
Two ways, per Shop's own help center. Orders populate automatically from connected email accounts or Shop Pay purchases, and a shipment can also be added by pasting in tracking information manually if it never shows up on its own.
Why does an order show nothing in the Shop app?
Almost always a missing tracking number. Shopify's merchant documentation states that customers won't receive shipping updates through Shop unless the store adds a valid tracking number and shipping carrier when the order is fulfilled. No number attached, no map, no matter how the order was placed.
What does the Shop app actually show once tracking is attached?
A map with the package's approximate location in transit, the store name, current order status, shipping updates, and a delivery date estimate that updates as the shipment moves. Shop's own help center states plainly that delivery tracking isn't available on the Shop website, only inside the app.
Is Track with Shop something a merchant has to turn on?
No, it is on by default. Shopify states Track with Shop is activated by default for all Shopify stores, and a merchant only interacts with a checkout toggle to show or hide the tracking link on the order status page, not to enable the underlying feature.
Does a label bought outside Shopify still show up in a buyer's Shop app?
Only if the number makes it back into the order. Shopify's own documentation ties Shop tracking to a tracking number and carrier entered at fulfillment in the Shopify admin, so a label purchased outside Shopify has to be pasted back into that order before Shop has anything to show the buyer.