The drivers are already in your kernel, if your device dates back to around the end of 2016 or older. The driver does need firmware-iwlwifi, which can be manually downloaded and installed manually if you depend on wireless for your Net connection. https://packages.debian.org/stretch/firmware-iwlwifi

If your device is newer, then you will need a newer kernel and firmware-iwlfi package from backports.

Firmware is not a driver; it is microcode that the driver loads into the device to get it to run. The iwlwifi driver has a list of acceptable firmware files which is goes down until it finds one that it can load. Most of those were only used internally at Intel and never released to the public, so many Intel wifi user will get those warnings, but they are harmless if it can find acceptable firmware files.

Is your device working despite the warnings?