1. The 8GB "card" is probably named "sdcard" - that's internal storage, just a folder named sdcard (it's usually somewhere like emulated/0/sdcard). The card you plugged in, the 64GB one, is usually named something like extSdCard (and usually located at /mnt/extSdCard).
2. Where pictures are saved is set by the camera app. If the app you use doesn't have a setting for that use a different app.
A Better Camera is a pretty good one (it's free).
3. Apps install to internal storage, even if you set the default to external. (They install to internal storage, then pieces are moved to the external card, and links are left in internal storage pointing to where the pieces are. Not all parts of an app can run from the external card, not all apps can have any part of them run from the card, and if there are a lot of pieces smaller than the links, moving the app to the external card takes more internal storage than leaving the app in internal storage.)
As for flash, you can get external flashes that are triggered by the internal flash. That's been available in the camera industry for decades - almost as long as electronic flash has. They trigger and flash a lot faster than the shutter speed of the camera, so they flash while the shutter is still open. You can use them as additional flash, to extend the range (the internal flash on most cameras isn't any use past about 5 feet), side-fill - anything. (I've seen shoots with 5-10 external flashes to flatten the lighting out.) As long as you're not shooting a very fast exposure (and you usually don't in a phone), the flash-triggered flashes work fine.