I am a bit confused, I have "login" and "iCloud" as default Keychains, are they both synced to Apple's cloud? Is the "login" Keychain synced to the cloud the same way the "iCloud" is?
I don't want to lose access to the passwords in the "login" Keychain. If it's not synced I am looking for a way to have all passwords transferred to the "iCloud" Keychain
There are two types of macOS keychain:
Data Protection keychains support passkeys, biometric authentication, and the Secure Enclave, while file-based keychains support none of these.
Apple intends to phase out the file-based keychains in favour of Data Protection keychains, but this will probably take several years as 3rd party developers will need to update their software accordingly.
The "login" keychain is used for secrets related to your specific machine. Most of these aren't needed when you switch to a new Mac, but some items, like encrypted Time Machine backup passwords, could be lost when you switch. Apple recommends copying the login keychain from ~/Library/Keychains/login.keychain-db when transferring to a new Mac.
As @Gilby pointed out, if you migrate using a Time Machine backup the login keychain will automatically be copied over to the new Mac.
Some items, such as Secure Notes, can be copied from the login keychain to the iCloud keychain; this is worth doing if you use the iCloud keychain. You can copy them in bulk but you have to authenticate for each one.
iOS and iPadOS devices only have one keychain, the iCloud keychain (if used).
Sources:
The login keychain is stored on the device, whereas the iCloud keychain is stored on the Apple server.
Source: Apple Community Discussion "Keychain versus iCloud Keychain"
