use an open source password manager that supports them.
as others mentioned, there's BitWarden (cross-platform, self-hostable), but if you want something simple there's KeePassXC (and you can put the store file on a dropbox shared folder)
The cabal of evil behind the passkey project actively have KeePassXC on their naughty list fore deigning to allow users to access their keys, and specifically included in the standard the means to discriminate between different passkey vault providers. It is the opposite of an open system, and cannot, under any circumstances, be trusted. Do not use passkeys, tell other people not to use passkeys, and make sure to not let shills astroturf conversations about passkeys unopposed.
I resent that I need a special app to "manage" them. I want to know where this key is on my filesystem so I can back it up and edit it myself, not have to use some app to access it. My ssh authorized_keys is just a text file. I can "manage" it with something as simple as vim. Maybe KeePassXC and BitWarden give you that simplicity, if so great!