Apps on Endless OS use the Flatpak technology behind the scenes. Flatpak defines the format under which applications are packaged and installed, and includes safety and security technologies. Flatpak-format apps are generally available through Flathub, but can also be made available from other sources.
The Endless OS App Center comes preconfigured to obtain apps from Flathub. If you would like to use an app that is not available under Endless OS, you can try to contact the developer and encourage them to make it available in Flatpak format, distributed via Flathub.
If you wish to produce an app for Endless OS, you should make it available in Flatpak format. Since Endless OS is based on GNOME, using GNOME technologies to build your app would be a sensible choice, and GNOME Builder (available in the App Center) is a recommended development environment.
Endless OS is also able to work with the usual variety of Flatpak runtimes which also enable you to use other technologies if you prefer.
Please consult the Flatpak developer manual for more details.
After you have produced your app and made it available in Flatpak format, you can consider publishing the app on Flathub. Once your app is published there, it will automatically appear in the Endless OS App Center for all users.