They'd also need to know my full name (I don't have any sort of social media footprint online whatsoever and there's no way to tie any of my usernames to any real person other than my gmail address which i only use for sites like paypal/banks/amazon and has no relation to my other usernames, and my middle name(s) aren't included anyway), the passphrase (which is decently secure itself), what "sitename" means in this context, and what character is used to separate the components in the input string.
An example would be something like "JohnAlanDoeG*GAys*dUuma1d=CGwwwgmail" where "John Alan Doe" is the name, "*GAys*dUuma1d=C" is the passphrase, "wwwgmail" is the site name, and "G" is the separator. It would produce the password "74a86c4e499cc8375900232a89ae27abe595196b3d32f0edd750ff0874dc1baa"
Of course no password system is secure on its own, and should always be combined with things like 2fa where possible, but my system would protect against the most common way of accounts being compromised (one site's information being leaked and using the same passwords on multiple sites) by providing a way to generate very unique passwords for each site that are very difficult to bruteforce. You would have to know a lot about me and my system to even have a chance of breaking the system, which is out of reach of all but the most dedicated crackers and I don't have any enemies like that, and as I keep all my online identities separate they would only crack the site they know me from without even knowing what other sites I use.
I'm not claiming I'm invincible but this system is like a password manager but without the requirement to actually have a password manager which may or may not be portable and I don't have to physically store my passwords anywhere.