I have a project with a notification like "develop 10↓". Does this mean there are 10 files changed recently on GITHUB since the last pull? Where can I view the files that were changed? Can I pull them selectively? (only a few of them after revising each). Where can I review the before-after differences? What is a staged/unstaged file? Any good and straightforward tutorials, please let me know.
...notification like "develop 10↓". Does this mean there are 10 files changed...
The "develop 10↓" part is talking about commits, not files. It means you have 10 new commits on the server that you haven't merge yet on your local branch.
Where can I view the files that were changed?
You can view these files by simply look at the corresponding commit.
Can I pull them selectively?
Well you can merge commits one by one if that's what your asking
What is a staged/unstaged file?
The unstaged files are just the files with the last modifications you brought. The staged file are like a snapshot that you take at a T time, Git will store these snapshots under staged files (one snapshot/file at a time).
You can find an excellent and complete lesson about Git into this link https://laracasts.com/series/git-me-some-version-control