Answer 1

first you need to create a table with the correct columns(database schema), then insert rows with values, then run a query to get the password value using the user name as the key. then you compare the value retrived from pyqt ui with what you retrived from database. conplete a sql tutorial in sololearn, then find a sqlite python tutorial
also do not ever store passwords in database, you only want to store salted and hashed passwords using at least sha256