How do I use unmerge function in excel on Numbers app?

You are misunderstanding what the unmerge operation is. It does not separate text in a cell and place it into separate adjacent cells. Merging takes two or more cells and makes them into one bigger cell. Unmerging does the opposite. The contents will end up in one cell. What you want is either Excel's TEXTSPLIT function or Excel's Text to Columns "wizard". Numbers has neither of these unless it has been added since version 13.1 (the latest I can run). I think there is an AppleScript or a shortcut that has been posted on this forum that will split text like the Excel Text to Columns "wizard" does. If you need a formula approach, REGEX.EXTRACT can be used.

If your text is in column A, put this formula in column B then fill right to get it into C, D, E, etc. for as many words as you need. The word separator is "any white space character" which includes space, tab, form feed, line feed, etc.

=IFERROR(REGEX.EXTRACT($A,"\S+",COLUMN()−COLUMN($A)),"")

COLUMN()−COLUMN($A) calculates a number (1,2,3 and so on) for which word to extract. You can replace COLUMN($A) with a 1 if you like.

I am not aware of there being an unmerge function but there is an unmerge operation/command that can be used on tables. Select some cells that have been merged then use unmerge to turn them back into separate cells.