Endless uses Flatpack for applications, and it has it's own built-in app repository. There are no paid apps, and the number of applications is limited. That being said, there are some good, quality applications there.
Endless is a great OS for students, schools, or ordinary desktop computer users. The way the file system is structured makes it VERY difficult for ordinary users to muck up the system.
The interface is clean and modern looking, similar to ChromeOS. It's lightweight and fast. It worked on the touchscreen Lenovo convertible notebook I tried it on, but the virtual keyboard didn't appear when tapping into the address bar in Chrome, nor in LibreOffice, so it's a work in progress.
Endless OS is still early in development, but it has some heavyweight backers and appears to be in it for the long haul. It will be interesting to see where it goes.