Atlassian Confluence can be operated on three different hosting variants:

While the Server & Data Center deployments are completely hosted and controlled by the customer (for example, behind the firewall), the cloud variant is operated entirely by Atlassian.

Confluence Server and Data Center are basically the same product and differ mainly in the server architecture. This difference is not visible to the user.
On the other hand, Confluence Cloud is very different from the other deployments and should be viewed as a completely different product, even if it has a similar name.

The extensions (apps) in the marketplace must also be viewed in this context. The server version of an app is a completely different product than the cloud app. Manufacturers do their best to ensure that the deployments are as similar as possible. However, this is not always possible because Atlassian has limited the technical possibilities in the cloud. This can mean that a smaller range of functions is available, that functions are implemented differently or may not be included.