I'm a bit confused about these two terms git & gitflow.
I know that Git is one of the version control systems, and git workflow is something about 'master/develop/feature/release/hotfix branches'. But what on earth is the relationship between these two and what's the difference?
and are 'gitflow' & 'git workflow' the same?
Since git is very flexible and not enforcing or limiting you to use and workflow, the community have tried to define and recommend a different workflow which should be used for developing with git
First read this article: https://www.atlassian.com/git/tutorials/comparing-workflows/gitflow-workflow which will answer most of your questions.
regarding gitflow - it's a methodology/flow introduced by Vincent Driessen back in 2010.
Here is the full article he wrote and described his idea, and the good part of it is that he added Open Source scripting for almost everything he described there and today git-flow is the most commonly used workflow