You can't map an individual key to launch a program, at least not using anything built into Windows, and even if you could, it would be a bad idea. The Windows key is used for a LOT more than just opening Start (use it in a chord with the number row, the arrow keys, the Tab key, at least half the letter keys, etc.; it's part of a ton of shortcuts).
However, you can create your own shortcut chords, no problem. One option is to pin the programs to the Taskbar, then use the Win+ chord to launch the relevant program (for example, by default in Win10, Win+1 will launch Edge because it's the first thing on the Taskbar). Otherwise, you can try the "Shortcut Key" field in the Properties window of a shortcut. That field is restricted in what chords you can use - Ctrl+Alt+ is the usual - but is still more flexible and isn't dependent on how your Taskbar is laid out.