Do OS's other than Windows support Alt-codes? If I put in Alt+1 for ☺, would I be able to put the same code in on MacOS and Linux and get the same output?
Alt-code is Windows specific. Other OSes have various ways to input Unicode characters.
On macOS 8.5 and later you hold ⌥ Option then type the 4-digit hex value. Characters outside the BMP can be input via Character Viewer
On Linux there are 2 common methods:
- Hold Ctrl+⇧ Shift and type u followed by the hex digits.
- Enter Ctrl+⇧ Shift+u, release, then type the hex digits, and press ↵ Enter (or Space or press and release ⇧ Shift or Ctrl)
There are other ways that you can find here