Storing bitcoin and other cryptocurrencies by means of using airgapped devices is an agelong tradition, it's not something new at all, but still, I am glad that op is sharing this here for us to discuss about it, so that newbies who don't know a thing about airgapped devices can learn.
The truth is, when we talk about airgapped devices, some usually think or believe that only hardware wallets can be used as airgapped device for cryptocurrency storage, the fact actually is, even our mobile phones can be turned into an airgapped device for our cryptocurrency storage.
How?
Just get an old mobile phone that you intend not using any more, connect to the internet and install electrum, or any other open source non custodial wallet, create a new wallet, write down your seed phrase and keep it somewhere really safe, move as much of your crypto you intend holding for a very long time into the new wallet, after confirming receipt of the crypto, now disconnect the old device from the internet, delete all network access settings, remove the Sim card from the device, turn the device off and remove the battery, congratulations, you just moved your funds to a new airgapped device, now, all you have to do is, keep the mobile device somewhere safe, as well as your seed phrase, also copy/paste and save the wallet address to the airgapped wallet somewhere you can easily access it, so that in the future when you need to move some funds into that wallet, you just get the address where it is saved and move funds there, you can import the airgapped wallet into another device in view-only mode using the public key/wallet address, a lot of non custodial wallets supports importing wallets in view only mode, wallet like electrum, trust wallet, mycelium and a host of others.