Questions get harder or easier as you answer. In five minutes you get a level for listening, reading, grammar and vocabulary separately.
Will it notice if one skill is far ahead of the others?
That is exactly why the four are measured separately. Someone who reads well but cannot follow speech needs different practice from someone with the opposite profile, and a single overall score hides that.
Does the result carry over if I sign up?
Yes. It becomes your level record, and your curriculum starts from that point instead of asking you to take everything again.