Facebook Autocomplete

Suppose that Facebook (Meta) wishes to implement an autocomplete feature for its search bar that will help users to direct to a page where they will get the result they were looking for.

For example: Instead of writing the whole word, by typing just “ap” a person should get in the autocomplete menu the desired word and then when he/she selects that word it will direct them to any desired page/group/person.

How do you design the auto-complete and how you measure the effectiveness