Best practices implementing list of objects in Java

I'm currently working on improving some old uni assignments moving them from serializable files to any other form of storage, mainly SQL Databases. I understand the concept of relational database design and the similarities with OOP Classes, however, I'm not entirely sure how to approach this issue from an OOP design perspective.

Right now I have a Hotel class with a List of Rooms as property, each Room has a list of Guests as property (full code here)

Back when using files I could mark these classes with the Serializable interface and store the parent object in a single file. But when using relational DB, I store each list as a single table and use separate queries to obtain the corresponding results. Same goes for the add() operation: with databases, I can do something like Guest.add() and add all the required fields directly to the database, whereas with my current design I need to call Room.getGuestList().add() (or a similar approach).

I totally understand that neither of both approaches is ideal, as both classes should be only worried about storing the data and not about the implementation of an add method, but even if I separate this in a single class, shall I still define a List property within each class?

I'm pretty sure I'm missing a design pattern here, but I cannot find the one that would solve this problem or maybe it's just that I've been taught wrong.

Thanks for your answers