I can’t remember exactly. There are two app stores and Discover is one of them. If I was you I would boot into a live or virtual environment and check it out. I think it’s explained in the welcome screen.

Sorry I can’t remember.

Correct. Just use Signal. Problem solved.

Discover in Kubuntu works a little differently than you would expect if you used KDE elsewhere.

You are right, the app management philosophy for Kubuntu is not quite right. Canonical put their effort towards providing Ubuntu with Gnome.

I had very similar issues, and since I wanted KDE (so many machines in my house already using KDE and I’m starting to get really comfortable with it) and I needed to use a specific app without containerization and the developer only makes a deb and rpm, I ended up trying Fedora with KDE. The DE bundled there is quite unadulterated, it’s basically a near latest stock KDE desktop. It was very seemless, you almost need to know nothing specific about Fedora, just use you muscle memory navigating the desktop.