and smartphones are more secure for storing Bitcoin than laptops and desktops.
This is not true, a mobile phone poses more security threat than a computer for obvious reasons, i.e. their OS. However it still depends on the user's opsec and how safely they use their device, because a desktop wallet run in an unsafe environment is more vulnerable than a mobile wallet run in a safe environment.
Actually mobile OS usually have better security by default compared with desktop OS. For example, Android offer Application and Privacy sandbox which limit what an application can do.
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Securing Bitcoin seeds is a very serious task. I had a special notebook in which I wrote down the Bitcoin seed. And it was very convenient for managing your satoshi.
I like the analog world more than the digital one. But now I constantly live with a girl and my notebook had to be destroyed (much to my regret).
I burned it. Because girls are very curious. Therefore, my girlfriend would probably sooner or later discover a notebook with private keys, passwords and seeds. And I’m not ready to share such information even with my girlfriend.
Even if she couldn't figure out what it was, she could copy the information and show it to third parties.
From this I concluded that storing seeds on paper is not always (unfortunately) the best option.
That's why many OS offer feature called full disk encryption. For example, bitlocker for windows and LUKS for linux.