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One can use operators such as switchMap or mergeMap to switch to a different Observable when a certain condition is met or event occurs. For example, if we have an Observable that emits user login/logout events, we can use switchMap to switch to a different Observable that fetches user data when the user logs in:

import { switchMap } from 'rxjs/operators';
import { getUserData } from './user-data-api'; // function that returns an Observable

loginLogoutObservable$.pipe(
  switchMap(loggedIn => {
    if (loggedIn) {
      return getUserData(); // switch to new Observable that fetches user data
    } else {
      return of(null); // switch to Observable that emits null when user logs out
    }
  })
).subscribe(userData => {
  // do something with user data
});

In this example, we use the switchMap operator to switch to the getUserData Observable when the user logs in, and switch to an Observable that emits null when the user logs out. This allows us to run code that depends on the user data only when the user is logged in.