example.ts

export class Bungo {
  constructor(private service1: Sevice1,
              private service2: Sevice2) {
  }

  findAllRowsBasedOnOtherRows(otherRows: string[]): string[] {
    this.service1.findAllRows$()
      .pipe(
        flatMap((service1Rows: Service1Row[]) => {
          return this.service2.finalAllRows$()
            .pipe(
              // filtering
              map((/* some _input from previous operation/s */) => service1Rows.map(/* do something here based on _input */))
            )
        })
      )
      .subscribe((service1Rows: Service1Row[]) => {
        // Ta da! Mutated Service1Rows[] based on some additional requested information
      });
  }
}