Before opening the page where you want to autopopulate, retrieve the database object you want to populate with, create new object, and if you want to auto populate, use the values of the retrieved object to fill the new object with and adjust the new object according to your wishes.
Or you could look at the autocomplete widget.
Or put an onchange microflow on the on change of the field and retrieve data from there based on the input of the user. And return the data you want.