How to properly deploy and work in a MERN project?

Question

Software Deployment

I will be deploying a MERN project for the first time. I have a number of questions, I will be grateful for the help:

- When I deploy to Heroku, where is mongodb transferred if I have it on a local server? Will I have 2 bases, one for development and one for production?

- How can I push updates in the future, for example, I did something on the local server, then git push, and then how to commit the changes to heroku?

I would like to read the full process of work from the moment of development to deployment and further updating on the local, and then applying the changes on the server.