How do I understand the code structure and contribute to open source project?

How do I understand for example this project:

https://github.com/gatsbyjs/gatsby

The code structure of the project is:

.circleci

.github

.yarn

benchmarks

docs

e2e-tests

examples

integration-tests

packages

peril

plop-templates

rfcs

scripts

starters

There's also contributing.md but this doesn't really explain on how I could add code to the project, I also am not sure how I could understand what each thing does in the project?

On a youtube video he says you can search for issues using githubs search function, but i'm not even sure how I can download an issue, fix it, then reupload, do I have to download the whole project then just reupload the issue.

I want to get stuck in on github and contribute but am stuck. I've found a few projects but am not sure how to contribute to them.

Edited November 19, 20205 yr by NullDrone