A question_attempt is an attempt at one question.
A question_usage groups together all the related question_attemtps for a particular use of questions, e.g. a quiz_attempt.
See https://docs.moodle.org/dev/Overview_of_the_Moodle_question_engine#Database_tables
(I know question_usage is a bad name, but I worked on this code for months withough being able to think of a better name, and I still have not thought of a better name, even though it is too late to change it now. We just have to live with it.)