for me, an OS is a set of tools to facilitate interaction with a computer
and if we take “operating system” literally, this definition remains accurate
it’s quite controversial, but i like this wide definition because it implies that an OS can be installed on another OS and it’s more fun
so for me ynh is an OS, but it depends on your definition
With this logic, Debian is just an application because it requires the Linux kernel
Strictly speaking, Linux is the real operating system in the technical sense, because that’s the layer doing the interface between hardware and software
Debian is “just” a bunch of pre-configurations and programs to be able to achieve useful everyday stuff with Linux which in itself is pretty useless otherwise. But in everyday conversation, Debian is the operating system, because for any practical discussion, nobody cares that you run “Linux”, it is way too vague and broad - i mean maybe your washing machine runs Linux. If the topic of conversation is “I want to install Firefox !”, nobody cares that you run Linux, because the exact step to be able to talk about how to install Firefox will depend if you’re running Debian, Archlinux, Fedora, etc …
Same sort of thing with YunoHost, you can say “I want to install Nextcloud !” - “What OS are you running then ?” - “Debian !” - but if you really follow a Debian tutorial to install Nextcloud on YunoHost, you will likely break everything and be completely lose the benefits of YunoHost which already does the heavy lifting for you and more stuff under the hood than you realize. Hence the OS is YunoHost.
In everyday conversations, the OS is what defines the paradigm under which you interact with the machine to be able to do useful stuff
When I talk about yunohost elsewhere, I define it as “a debian based distro with a webui”. So it’s not an os or an application, as I see it. It’s a distro like the other ones, with a clear use case. That’s my point of view.