Zotero is open-source software and therefore, it is free and gets updated regularly. You can set up Zotero to automatically download these updates in preferences. See 'Setting Preferences' for information.
With the free version, you have 300MB of personal storage space but it is possible to pay for a premium plan which gives you more storage space.
There are several iterations of Zotero:
You can synchronise your Zotero library across these different platforms and devices if you use the same username and password.
A free Zotero account gives you:
To see how much storage you have left: