I want to implement OTP based login for my application can someone recommend good services for this use case? Are there any free services? I am using Nodejs+Mongodb

Check out supabase

Implement short-term password, rather than one-time password and it becomes a lot simpler to implement as you don’t need any storage at all. Just produce a deterministic code based on (userid, timestamp, action) and verify it when user submits it.

Any example which you can share? did not get you completely