It's actually "Unlocking this will allow you to purchase and add one detailed surface scanner to your storage." Because you still have to purchase it.
Not really, because you don't purchase it in the classic sense (with credits). The "unlock"
is the "purchase" (with materials) at which point it becomes available in Storage. Whether in lore this is an exchange of goods by the Tech Broker, or they make the thing with the bits you give them, is unclear. Use your headcanon of choice, I guess. Either way they end up in Storage, which is why you see them in the Outfitting screen. But the credit cost to install them is zero, because you already own them.
So is "Engineered" the key? If it says "Engineered" then you have to unlock and buy each time you want one?
Not quite, because the Sirius Heat Sink Launcher and Modified Mining Laser are also exchanged for materials each time.
If you are logged into an INARA account the page at
https://inara.cz/elite/techbroker/ differentiates between those items that are unlocked and those that are "made to order" as it were. But this distinction is only visible if you're logged in, presumably because INARA tracks which items are unlocked on an account-by-account basis. Just to muddy the waters further, INARA refers to non-unlockable items as "Purchasable". But at least you can easily see which is which.
