What are main differences between Git, GitHub and Bitbucket resp Gitkraken?

hello dear experts,

i am pretty new to GITHUB.

i just need to digg into all that sort of things. Git and mercurial(hg) - both are the actual tools for version controll. In fact both them can be called "Distributed Version control system-tolls", While git and mercrurial can be called true vanilla version controlling 'tools' which can be used for the creation, management and version controlling of git/hg repositories,

well - i have heard that we can use git with (on) Github and bitbucket. Is this true: Can i run git on both systems GitHub and bitbucket. That would be great.

The question is: Which Git GUI should I use with GitHub, Bitbucket or GitLab? A friend of me told me that i should run (use) GitKraken. but Wait: Gitkraken is told to be a commercial project that only wants to make mone.y

but technically spoken gitkraken has some interesting features. GitKraken connects to all 3 to clone/add repos, create/view PRs & more. Free for Windows and also Linux! So the question is: should loook at gitkranken or stay with GIT, Github and Bitubucket- and leave gitkraken as a tiny commeercial project that is not worth looking at.