Apps install with one click, update themselves, and never touch the system files.
The quickest way: Steel Apps
Open the Steel OS Portal from the taskbar.
Click the Steel Apps tab. Apps are grouped: Everyday, Work, Creative, Games and Streaming, Developer.
Click an app, then Install. A small window shows the download; press Enter to close it when it says so.
The app appears in the start menu.
Thousands more: the App Store
Open App Store from the start menu or the taskbar. The Curated page starts with the Steel Apps; Search finds anything else. Click Install.
Removing an app
In the Portal: click the app, then Uninstall.
Or in the App Store: Library, find the app, Remove.
Already installed for you
Firefox, LibreOffice (Word, Excel and PowerPoint files), OBS Studio, Kdenlive, Obsidian, Moonlight and Zed install themselves a few minutes after your first start, as long as you are online.
Windows programs (.exe files) don't run directly. Most have a Linux version in the App Store. Windows games run through Steam or Lutris; see Gaming.