difference between fork and branch on github

If you fork a project, you are making a copy of the whole project to your git hub account. you are not coping anything to your PC

To make a copy in your PC you have to clone it and pull all the stuff and you will got all branches & code of that project

All branches on GitHub will be copied in a fork. (Obviously, this doesn’t include branches that were never pushed to GitHub in the first place.)

But a fork is a GitHub-to-GitHub operation; nothing is copied to your PC. It’s not quite the same as a Git clone. If you mean to ask “what’s copied when I clone a project?”, see the manual for git-clone(1).

Fork is a clone on the GitHub side (it clones everything).
When you are cloning a repo, you are getting the all history of said repo, with all its branches.

Even though you can in theory change the default branch of a remote repo, a clone from a GitHub repo mainly look for the master branch. Meaning to change the "default" branch a GitHub clone will get, you need to rename the master branch.

Think of it this way:

The repo[sitory] corresponds to the collaborated work of the team across one or many branches. All contributors have their own copy of it.

Each fork of the main repo corresponds to a contributor's work. A fork is really a Github (not Git) construct to store a clone of the repo in your user account. As a clone, it will contain all the branches in the main repo at the time you made the fork.

Each branch within the fork and/or in the main repo can correspond to several kinds of things, depending on how you want to work. Each branch could refer to a version of the project but can also correspond to different channels of development, like hotfixes or experimental work.

The pull request (in the GitHub ecosystem) corresponds to the task. Every time I want to contribute an isolated finished task to the main repo, I create a pull request corresponding to the commits made in that task. These commits are pulled from either my fork or my branch to the main repo.

A commit is a set of changes to the code. This is one of the most interesting things about Git. You don't transfer files, you transfer logs of changes.