The concept you’re referring to is how to effectively set up a relational database (ie. User A is connected to Thing 1, Thing 2 and Thing 3). On LearnTo (my tutorials site), I have a good number of tutorials covering this concept.
I might suggest watching the lesson (linked below) from my public library (on Youtube) about how to build a grocery list app. (One of my go-to’s for getting started with relational databases). The lesson takes you through the progressive stages of thinking about a database (first with simple text-based lists, then into more complex relational setup_.
While it may not seem exactly relevant at first, if you can build a grocery list app, you can build just about any relational database you need to in Bubble.