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

SuperTokens.com

I have a question about SuperTokens: "Is there any way you change the UI of the Sign up/Sign in page in React.js?"

There are several ways to do this. You can change CSS, you can change parts of the react component tree, and finally, you can run Js to manipulate the form however you like.

https://supertokens.com/docs/thirdpartyemailpassword/common-customizations/styling/changing-style

https://supertokens.com/docs/thirdpartyemailpassword/advanced-customizations/react-component-override/about