Yes, Autodesk Fusion primarily operates as cloud-based software. It requires an internet connection for most of its functions such as saving designs, collaboration features, and accessing the Autodesk cloud library.
However, Fusion does offer a limited "Offline Mode" that allows you to work without an internet connection for up to two weeks. You can activate Offline Mode by clicking on your account avatar in the upper-right corner of the software and selecting "Work Offline."
While in Offline Mode, you can edit existing files and create new ones. However, you won't be able to save changes to the cloud, use cloud-based features, or access files stored in the cloud. Also, any changes made in Offline Mode won't be synced with the cloud until you reconnect to the internet. For more details, you can refer here.