Nextcloud is a server, not an app. It does have mobile apps and a desktop sync client, but these are only clients.
Technically you could install it on your desktop if it’s *Nix or if you run a virtual machine, but this is less than ideal.
That being said, I personally prefer the Docker method, and if you run this on a Linux desktop, Docker might help to avoid potential conflicts with other software on the system. If you’re interested, I wrote a setup guide for it.