SharePoint automatically creates a query suggestion when you have clicked a search result for a query at least six times. For example, if you have entered the query word "test" and then clicked a search result six times, "test" automatically becomes a query suggestion.
Automatically query suggestions are generated daily for each result source and each site collection, so the query suggestions can be different for different result sources and site collections.
If you want to display query suggestions immediately, you could import suggest phrase from text file.
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