On Gitweb, we can see the complete file of a commitId. On Emacs Magit, I can get to that commitID and see the diffs. Is it possible to see the complete file for that commitId via Magit?
Yes you can. I'm not sure of your workflow but I'll explain with the following use case.
I open the magit-status and I see the commits log with ll, so I see commit ids on the first column. I choose a commit to see and I press enter: I see the diff for each file of the commit. Now I choose a file and press enter: I see all the file as it was at that specific commit (the buffer is named something like "file-name.el~master~2~"). Does that answer your question ?