If you are trying to group rows based on the "places" columns, Microsoft/SharePoint Lists offer that capability out of the box.
There is no way to do this in column formatting that I know of. Column formatting can only reference the current row's fields, not values in other rows.
If you want a column that contains all of the values entered in all rows, I would use Power Automate to loop through all rows to build a collection of unique values. Then update every row (or row last modified) with the collection of values.
If you have a specific use case, it might be easier to give you a better solution.
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