SuperTokens is an open source authentication solution offering features like: Different types of login: Email / password, Passwordless (OTP or Magic link based).

SuperTokens.com

does anyone have a concrete example of advanced customization for the React components?  either advanced customization or using rolling your own?  that would be very much appreciated.

https://github.com/supertokens/supertokens-auth-react/tree/master/examples/with-sign-in-up-split-emailpassword

https://github.com/supertokens/supertokens-auth-react/tree/master/examples/with-emailverification-then-password-thirdpartyemailpassword

are there any examples of using a different UI component or components?

there is a blog about it for social login: https://supertokens.com/blog/adding-social-login-to-your-website-with-supertokens

we're exploring all options to make sure that our customers UI requirements are exactly met

You can build your own UI and call the APIs directly.