SQL databases rely on relational algebra and normal forms. Document databases use comparable math, just starting from non-1NF instead of 1NF. A 1986 paper, "Theory of Non-First Normal Form Relational Databases," introduced nested relations, expanded relational operators, and a query language called SQL/NF. Great concepts to understand document database modeling and its use. To make it easier, I mapped each concept to its modern equivalent, MongoDB, with examples:

https://dev.to/franckpachot/non-first-normal-forms-1nf-and-mongodb-an-alternative-to-4nf-to-address-3nf-anomalies-17i8