Not a technical answer but when we started up the system (zx 16k) we were in a prompt. We would add commands with line numbers. After each line number the list of possible commands were embossed on the keyboard and you would start with that (if, peek, poke, etc). What you could complete was limited by that. Edit: BASIC programming

That was not a very good description so try this: https://www.usebox.net/jjm/notes/basic/ (scroll down for keyboard pic and also some code). Or this video : https://youtu.be/zgjGsNS6a0Y?t=167