I often google historical background for different sageuks to know how their world worked back then. Also, I like to trace family trees to learn how characters from different dramas were connected and how different drama portary same people. Or to spoil for myself if any of characters is getting married/divorced or die
For example, I googled
4. The Red Sleeve - I was wondering how realistic main romance and some information on these two people. Even though their story was romanticized, some facts are true like when she was promoted to Bin rank the king choose prefix Ui for her himself. Or that she was the only woman he loved among his wives. Also, the fact he is considered one of the greatest kings.
5. Tomorrow With You - it confused me that FL went to register their marriage AFTER the wedding but I searched and learned that it's quite common in Korean that wedding and registration are two different events and people often register their marriage after the formal ceremony, and some couple may throw a wedding but do not register their marriage at all or register it years later. In my country these two events normally happen on the same day, or registration may be BEFORE the wedding.
6. Miracle in Cell No. 7 - I searched background information on the death sentence in Korea and learned that they actually had it up to 1997 and that ML's story could take place irl and that the story was based on a real life case
Bonus: Military Prosecutor Doberman - one of the characters got the death sentence that wasn't executed (the character kept living). I searched and learned that death sentence still exists in SK but means life imprisonment with no parole rather than an actual death