Despite being allowed to pick any datastore, pick MySQL or PostgreSQL and get acquainted with transactions and locks. That was only the real difficulty, if you pick something else it'll weight against you. Both coding and system design was related to SQL. React question was easy in scope but difficult to complete within 45 minutes or less. They expect you to write a full search tag with autocomplete with CSS. I completed the exercise without CSS but my lack of experience with React weighted against me. CSS was just a bunch of flexbox and a couple of borders and margin but didn't have the time for it.