All users get 1 Terabyte (TB) of storage in their OneDrive folder.
Yes, OneDrive allows you to share files and folders with other employees and students. You can also share files with external contacts but they require a OneDrive account to receive the file. For a tutorial on how to share checkout this youtube video
Open up OneDrive in your browser and find the desired file. Right click on the 3 dots by the file and select ‘Share’. A new box will open and show people who you are currently sharing the document with.
Yes you can. This is known as "co-authoring" a document in Microsoft terms. Simply share the file with someone else (through OneDrive's sharing options or through Teams) and once you both have it open you can edit and see each other's edits in real time. For more information on this see Microsoft's info here. Or a great youtube video on the subject here.