You understand it correct. You are not allowed to use an engine (or endgame database) during the game.
But (in Daily) you can use your notes, books, videos, game databases etc. - also where these has been made using an engine (most people use an engine to analyse their finished games and put their conclusion in notes, most books are checked by an engine, some games are played by engines, etc.). This will help you some of the way (and some lycky times give you an easy win by a trap etc.) but most of the times you will soon be on your own :-)
Is it really okay if I'm studying my own notes in a correspondence game? There are a lot of analyzes and records of master games and my own games of the respective opening on chess.hirner.at There is of course no software that can calculate errors or best moves in the current game, because they are my writings, but they are records for which I used a software like Stockfish. So I have doubts and ask for a brief explanation, because of course I want to play fair and don't want any advantages over others.