Will iCloud Work on all my devices including Mac?

If I upgraded to iCloud+ on my iPad and I’m using the same Apple ID on my iPhone my iPad and my MacBook. Will the iCloud+ 2 TB think and be able to be used on all three? Also does anyone have any tips on why my MacBook Air will not let me access all of the features it is a mid 2014 model. Also on my MacBook I don’t know what I did but I used to just have one to mean login when you turn on the Mac but now I have two? I am scared to delete one or the other. Because I have a lot of court documents on there and now I don’t know which one to log into it keeps telling me that on a lot of documents I try to open that I don’t have access to view that document?