Not sure if this is really what you're looking for but you should be able to find something in one of these:
/node_modules/next-auth/
- core/lib/email/signin.js
- core/pages/signin.js
- core/routes/signin.js

Really, sign in pages come from the provider. This is the functionality that next-auth handles

For example, if you're using the Keycloak provider, the keycloak signin page comes from keycloak, but next-auth provides the backend functionality to handle token, session, callback, etc.

Sorry I meant the page where you select your provider

this