It's pretty much the same process as described earlier in this thread, except instead of using a second computer as your airgapped device, he is using a phone.
This is certainly better than just sweeping your private key in to a hot wallet, but it is not as good as using an airgapped computer for a couple of reasons. A phone in flight mode is not truly airgapped, and studies have shown it still pings for cell towers, it can still send location data, it can still leak via WiFi and Bluetooth, and so on. A phone is not truly airgapped unless you open it up and start ripping out the antennas and WiFi modules. You also are not starting on a clean device, and your phone could have any type of malware on it already. Even if you factory reset your phone, you aren't starting from scratch, but rather starting from whatever bloatware and spyware your carrier/manufacturer has pre-installed, and with their (usually) closed source OS.
An airgapped phone is an OK middle ground, but it is not as good as using a fully airgapped computer with a clean install of an open source OS.
Since you're looking for the safest way to do it, I agree with @o_e_l_e_o's answer above. You only get closer to the highest safety/security possible when you eliminate all the potential risks and flaws out of your method. And so far, the only remaining potential security threats is someone physically tampering with your airgapped device which is imo a quite low probability unless you're being watched by intelligence agencies or have some bad actor around. That is unless you somehow mess up yourself and accidentally enter the airgapped device's seed into the online PC, which I believe is something that has a low chance of happening as well.
I would not use a phone because it has just too many backdoors and unknown/non-removable components. A PC is quite modular and you can easily remove or add anything you might or might not need. For me, privacy & security are not only interests but also a fascinating hobby (and now addiction, I guess). Even if you aren't followed by 3-letter agencies, I would still recommend to follow the best security practices as it's more than worth it over the long term. And since it's both fun and safer, why not?
Absolutely I agree with you both. I just posted the video link to illustrate the general process as it was all I could locate online. I'd never use a mobile phone with downloaded app.
Thanks.
