Unlike Search Dictionaries, which only affect classic SharePoint search, Query Suggestions customization affects both classic and modern search, since they can use the same default result source (you can think of a result source as the content that you'll be sending your query to search against). Query suggestions (also known as predictive text or type-ahead words) appear below the search box as a user enters their query words. SharePoint Online creates a query result for a suggested term when you've clicked a result at least six times.
Query suggestions are populated daily for each result source and per site collection, meaning that suggestions can be different across the organization. These automatically-generated suggestions for the default result source are used in both search experiences.
In addition to automatically generated query suggestions...
