The most valuable digital data I own isn't a single thing but rather the unity of everything, because my magnum opus is I have the world record for the most websites having signed up for. Think of any website in the world and I'm probably there, almost always with one account. And you may recognize me on there if there's a chance you see me talk about this on there too.
You didn't think world records had their own wiki pages? That's how I was able to grasp confirmation that I did it and that I have the world record for the number of websites having signed up for. Inevitably it's also engraved in the iterations of the wayback machine.
I have notes detailing literally thousands of websites where I have usernames and passwords, some even on the deep web, some even on consoles, and so forth. Doesn't matter the device, the browser, what is required to access the site, etc.
I rarely use the same names on more than one website, that's not something I got into the habit of due to the different systems on every website. Half of them are random while the other half follow a naming theme, where I make a username based on the things Stingy says are his in the Mine Song in Lazytown (thismailbox is mine, and thistriagonalsign), which helps with memory (I also sometimes vary in my used nicknames depending on the site).
Knowing you have a world record is something that just sticks with you, it's like having a kid (no I'm not a mom yet). I am grateful for my ability to keep the record.
Just fyi, if you have a password mnemonic, you should NEVER share that with anyone. If a malicious party learned of it, knowing the rules by which you formulate your password can help them generate a highly-targeted dictionary attack with average success runtimes orders of magnitude lower than an unassisted cracking attempt. Moreover, knowing the mnemonic basically lets an attacker extrapolate ALL of your passwords.
No, what I describe only applies to usernames, never passwords. For that, it helps to have several hiding places where you can hide different parts of a printed guide.