Yes you can build extensions to Oracle Cloud Applications - you use the APIs they provide to access the data.
There are several tools that can help - depending on your application, your skills and your needs.
One option is Visual Builder Cloud Service - here are a few videos showing the experience of building extensions with that tool:
In addition Oracle JDeveloper with Oracle ADF can be used to build extensions to Oracle SaaS and then you can host these extensions on Oracle Java Cloud Service - SX.
Thank you Shay for the information.
Will check out the links and get back.
Can you add some comments what the document suggests by the following statement, in the Functional Admin guide regarding Design time customization
Note: These customizations and extensions aren't available in Oracle Cloud implementations
The "not available on cloud" means the type of customization that involves directly modifying apps pages in JDeveloper (using MDS layered customization). In the cloud you don't get access to the SaaS code to load it into JDeveloper - but rather use the composers to customize existing pages. You can still use JDeveloper to build new pages.