Vimscript - how to make menu/selection similar to `z=` and `g`

Hello fellow Vim users!
I began learning vim script from the most excellent resource: http://learnvimscriptthehardway.stevelosh.com/

I noticed 2 things "vim can do", but I don't know how to re-use for my scripts:

1. when the cursor is on a spelling error, you can press `z=` to get a list of possible corrections. Is there a way I can get that behavior and use it for other things?
Naturally, I wouldn't use that for spell correction - but for let's say a choosing an option from a long list.

2. when using ctags with vim, you can press g<Ctrl-]> to get a (colored) "menu" with a list of matching tags and a prompt to jump to selected tag (just a note - the menu and prompt is shown only if there's more matches for the current tag).
Now, the same as above - is there a way to get this exact menu/prompt for other scripts and other uses?

I couldn't find anything useful on google for this - I think I don't know the right terms.
But a pointer to a help section or a related link would be most useful!

Thanks

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