how to query a field for all inputs?

This use to be more common, and something i never understood. Sometimes, when clicking in a field on a site; the field will put the terms, keywords &or phrases into what i can only think of as being in cache. And it always seemed relatively random that i would find a field having all those inputs. sometimes more than dozens, based on what i figure to be the time between flushing browser cache, however i don’t remember this to be browser specific behavior, as I believe I remember it occurring in other areas of the 3 main systems being ran. I never understood how i was able to get those fields to show those lists, without just typing a letter that might begin or be in those inputs. this occurred mostly in search fields, but also it use to happen in like an email address field. I understand with how security exploded, causing many developers to sanitized search fields but occasionally it still happens, like in user login fields, but lately those fields seem shared. brcau6se it populates with usernames that don’t work with the site it’s showing up in. is their a way of like querying those fields directly, so i can just pick keywords and such from a list, like a key combo specific to this inherently designed behavior?

It just dawned on me to ask: Is this SQL / MySQL database stuff?