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

SuperTokens.com

Hey, I was trying to edit the signup error msg: "Email already exists. Please signin instead.", but I havent figured out where this can be overwritten.
Any help would be appreciated.

Did you see the language translation section in the docs?

You can use that feature to provide a custom message for this error type

https://supertokens.com/docs/emailpassword/common-customizations/translations

I see that "plain javascript" (using vue3) points to the SDK, but can I use the same setup as react in the Init() function for web-js?

No. You will have to put this config in the supertokens-auth-react config