They’re accessible outside of a browser, via a “keychain”, and the entire OS is built to use this keychain, which also syncs appropriately among your devices.
On iOS, it’s Settings > Passwords. On MacOS, it’s Keychain Access, which looks like this:
There is also a UI in Safari itself, which on MacOS has added some advisory features, including easily guessed, seen in a data leak, or used on multiple sites:
