That is how it works if you have Messages sync enabled. Other MFAs are also synced on Apple devices: TOTP and Passkeys are synced via iCloud Keychain to all iPhones and Macs using the same iCloud Keychain account.
I believe google synced TOTP and Passkeys between Android devices using same google account, i did not test this though.
Obviously one can disable sync, but imo synced MFA is what most want anyway.
That is how it works if you have Messages sync enabled. Other MFAs are also synced on Apple devices: TOTP and Passkeys are synced via iCloud Keychain to all iPhones and Macs using the same iCloud Keychain account.
I believe google synced TOTP and Passkeys between Android devices using same google account, i did not test this though.
Obviously one can disable sync, but imo synced MFA is what most want anyway.