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

SuperTokens.com

Do I have to run supertokens.init() for every api route in Next.js if I am not deploying in a serverless environment?

U don’t have to run that if not deployed in serverless env.

Where would be the appropriate place to call this in a non serverless env? in the _app.tsx file?

Yea. If you knows it’s the backend and not the frontend. So maybe call it when something like process.env is not undefined. Not sure what the right check is

I think it's just typeof window === 'undefined'