HJL,
Downloading YouTube on Android. Remember micro-SD cards are interchangeable and inexpensive, so get a phone with a slot and make backups (and put them around various places). Often large (128Gb+, often U3 for GoPro) capacity cards will go on sale at least once a month, so I look for them and stock up. I’ve seen these as low as $15, limit 2. Sometimes there are excess stock or closeouts on prepaid, and I didn’t have to activate them though there was a trick to bypassing that screen. Most can be rooted. I have several, so it pays to look through the online bargain bin, particularly the refurbished section for things like tablets. One app to get is http://maps.me/en/home; they have OFFLINE maps (even if the Internet or GPS goes down, it works though it won’t track). It’s detailed and of the entire world. And change can be good for hiding them, which should also act as a Faraday cage.
M-disc or even regular DVDs or blu-rays aren’t electronic themselves, so I tend to keep a second backup on them. Generally, it is better to keep backups on at least two different media types, so I use these. I also have a media safe, as normal safes will keep paper from burning, and these have extra insulation to preserve magnetic and optical media and electronics.
One final thing, for the price of the fancy Ziplock Faraday bags, you can buy enough aluminum foil at the dollar store or Amazon Prime Pantry to wrap a vehicle, much less phones. I can also get two 33-gallon galvanized steel trash cans for an extra layer of protection. If you are worried about screen scratching, use a soft cloth or lint-free paper towel. – T.Z.