Sure, here's an example of how to join data in RxJS to display the name associated with an id:

Assume you have the following two arrays:

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const users = [
  { id: 1, name: 'Alice' },
  { id: 2, name: 'Bob' },
  { id: 3, name: 'Charlie' }
];

const items = [
  { id: 1, itemId: 123 },
  { id: 2, itemId: 456 },
  { id: 3, itemId: 789 }
];
And you want to display the name associated with each itemId in the items array. Here's how you can do it using RxJS:

Import the necessary RxJS operators:
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import { of, from } from 'rxjs';
import { map, mergeMap } from 'rxjs/operators';
Create an observable for each array:
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const users$ = of(users);
const items$ = of(items);
Join the two observables:
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items$.pipe(
  mergeMap(items => from(items)),
  map(item => {
    const user = users.find(u => u.id === item.itemId);
    return {
      ...item,
      name: user ? user.name : 'unknown'
    };
  })
).subscribe(result => console.log(result));
Here's how this code works:

items$ emits the items array.
mergeMap flattens the array of items into a stream of individual items.
map processes each item and looks up the corresponding user object using find.
If a user is found, the item is enriched with the name property from the user object. Otherwise, it is set to 'unknown'.
The final result is logged to the console.
You can adjust this code to fit your specific use case. For example, if you want to display the joined data in a template, you can assign the result to a component property and use *ngFor to iterate over it.