Edit: For the sake of completeness, the deleted parent suggested that iPhones are unable to be opened because they're glued up, and doing so (to reseat the battery) would void the warranty
Nope.
(Former) Apple Retail 'Genius' here. iPhones are trivial to open up to get to the battery, provided the right equipment (tore screwdriver and suction cup + grip), and disconnecting the battery is just a simple tab/plug to lift up.
Because this isn't a 'modification' (Apple warranty forbids unauthorised third party modifications) and is exactly what they would do in Apple Store anyway, you're safe to do this yourself without voiding your warranty, as long as you know what you're doing.
How was customer data erased or prevented from later being perused by employees on those "bricked" phones customers have swapped out? Because customer data would still be intact if such a device were looked at by engineering or others with sufficient test equipment, especially on a device that didn't have a passcode.
Sometimes for an actual hardware service (say, replacing screen on iPhone), the customer will be asked to erase the data on the phone themselves before handing it over. This is to stress the point that they should have a backup, and so Apple can just quickly replace the phone if someone goes wrong with the repair.
But for something like reseating the battery, the main factor is they just don't care enough - the employees are trying to get through your appointment as soon as possible (customers seen/avg session duration is a metric they're measured on) and 99% of the time, they just aren't interested in the data on your iPhone to even try. Once they see the phone is powering up, they're done with it.
Then there's fear of retribution. Looking at customer's personal data is seriously prohibited, so you'll be in a lot of trouble if you get caught.
And of course, security. If there's a password on the device, there's simply no way for them to get further than the lock screen or access any data. Even if you're willing to believe a conspiracy theory that Apple technically can through a backdoor they've built into it (which seems highly unlikely given their crusading around this topic), there's no way in hell Apple would give the 50,000 underpaid paid Apple Retail employees access to a tool like that.
But at the end of the day, it's trust. "If an attacker has physical access, your system is already compromised". If you don't trust anyone else to handle your phone without trying to 'steal your data', then you should erase it yourself before handing it over.
Maybe I wasnt clear enough: what happens to these unfortunate folks that hand in bricked phones whom are unable to erase content? Refurbished by third parties, sent to engineering, recycled?
I am not sure about iPhones but opened all of my Macs before, sometimes to do some upgrades or clean up. It never voided warranty. All three had some work done by Apple. As long as you do not screw up yourself something during repair, it is fine.
I would expect majority of HN population to be able to follow step by step guides online.
