how can I look at all my third party subscriptions and change them to sign in with Apple????

Hi! I don’t believe there’s a way to do this, as accounts on third-party apps or sites are not managed by Apple. You will have to check in the settings of the individual apps or sites to see if you can change your account to ‘Sign in With Apple’.

Have a great day!

To use Sign in With Apple, you must initially use it when signing up or you must create a new account. Please note that some services allow the ability to add Sign in With Apple along with other methods.

See: Manage the apps that you use with Sign in with Apple - Apple Support