Before you can develop ownCloud applications, as with developing other software applications, you have to ensure that you have a working development environment.
To do that:
First, ensure that your development environment meets the minimum requirements
Then, create the core files that any ownCloud application needs
There aren’t many; all that you’ll need is:
PHP, with a minimum version 7.3.0 (though preferably 7.4)
A copy of ownCloud core
A working installation of ownCloud server
To find out more, read through the Development Environment section. When you’ve done everything that it suggests, you’re ready to begin developing an ownCloud application.