Salesforce + NodeJS + OAUTH Help Needed (connected app)

Job ID: 31031139

Budget: $10 – $30 USD

Hi
I'd like to make a NodeJS / ReactJS App on Heroku connecting to Salesforce.
Now I need support relating to authenticating.
This site could be an example of how it should work: https://fieldcreator.herokuapp.com/

Basically I'd like the user to go to the Heroku page and authenticate with Salesforce and grating access.
Then the app can access Salesforce (meta-) data as needed.

All tutorials on the web that I found relate to a "connected app" that needs to be created first in Salesforce.
But when going through the Login-Process on https://fieldcreator.herokuapp.com/ everything seems to be done in the background. The user just needs to login to his Salesforce and grant access.
I would like to have the same functionality!

Who can help me by explaining me how this can be done (showing me a tutorial / quick call / session)?
There is no need to program this for me (i will do it) but rather explaining me how this authenticating process can be done.

Thanks
Pascal