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Use Observables: Observables are a powerful way to handle asynchronous data in Angular, as they allow you to subscribe to changes in a collection of data over time. It is easy to combine multiple Observables to create a new data set.

Use RxJS operators: RxJS provides a range of operators that allow you to combine, merge, or transform Observables. The combineLatest() operator is particularly useful for combining multiple Observables.

Use Promise.all(): If you need to combine multiple Promises, you can use Promise.all() to wait for all the Promises to resolve before combining their results.

Use Firestore collection groups: If you have multiple collections with the same structure, you can use collection groups to query all these collections as a single collection. This can simplify your code and make it easier to combine the data.

Use a service: Creating a service to handle the combination of values from multiple Firestore collections can help keep your code organized and modular, making it easier to maintain and reuse.