-snip- I don't understand where are stored the BTC locally? Or maybe they are not stored locally? -snip- Isn't there a file somewhere in my local macOS that is my wallet and that I can copy to a USB key?
Open Finder, then click Go menu -> Go to Folder (Shift+Command+G) then type ~/.electrum look for the Wallets folder, there you will find your Electrum wallet files (by default). You can copy it to a USB flash disk or other media.
