Theoretical openings are those well explored and documented. Novelty is the first deviation from the theoretical opening.
How to study: Play the moves that are played most frequently in the database. For first few setups and then explore the lesser moves off that. E.g. e4/e5 and then Vienna.
After a loss, look at the database and the the computer eval to find the novelty and mistake1.
References
1.
Rozman, L. How To Learn & Study Chess Openings. at https://www.youtube.com/watch?v=6IegDENuxU4 (2020).