Gatsby is static site generator. it has three phases

1. Collecting data sources
2. Building
3. Deploying

Collection step

In this step it will collect the data from sources here the sources are

Databases

or

XML files

or

CSV

or

Text files

or

JSON

or

Wordpress

or anything.

Building step

Once data is collected then we can access it from source code with the help of graphql. Here we cannot mutate the existing data

Deploying

Once we are done with development we can build the app and we can host that build files in any static web host sites like Amazon S3, Netlify, Github pages, now.sh and many more...