Unique Database for Each User

Hi, I have a general understanding of how database works, e.g. creating types and entry fields, saving input field’s value etc.

I’m trying to build an app that allows users to create their own fields of something, and only see their own creations.
Take expenses apps for example, each user would be able to add, remove, and define their expense categories - transportation, entertainment, bills… etc.
Or take sales management apps for example, each user would have their own product/services stored in the system for later when they need to include in an invoice.
In short words, everyone has their own “mini database”, and my app will provide a page for them to check on and edit all their creations.

Now I understand that you can limit users to only see their own creation, but does that mean in my database, I (app builder) might have thousands of fields created by different users?
Is there a better way to manage this?
Would this be risky in the sense that if the workflow (or anything) goes wrong for some reason, users would accidentally see other people’s data (since they’re sharing the same database)?

Much appreciate it.

This is my current expenses and project status database for expenses, by the way, this should be default options and allows users to add their own.

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