On the desktop there is a link to 'Back to Game Mode'. Select it to return to the standard Steam Deck UI. You can also go back to Log Out to go back through the system menu.
You can use the track pad as a mouse. Clicking on the right touchpad is a primary click, clicking on the left touchpad is a secondary click.
Yes, if you're running Steam, apps and games in SteamOS desktop mode have access to entries in your Steam deck
Currently, this is a standard KDE Plasma experience in terms of software installation. You can use the Discover Software Center (on the taskbar) to install applications. Discover Software Center installs applications on the writable portion of your drive using Flatpak technology (more on this below).