RxJs observable data processing in Angular service

I am having some issues with working with the data from RxJs and other data in a service in Angular.

I am working with Angular 15 and standalone components, but also trying to follow some best practices for working with data.

My component is to show data from various sources. The issue I have is combining all of that data into a response from my service. I believe my issue is simply in not fully understanding the RxJs pipe, subscribe, map functionality.

In my example service, I can get the different parts working correctly, but I cannot combine the results into what I want.

I will omit a lot of the service that is not relevant to this task. Basically, my code is fetching data from a web service back end using Http, and getting that response as an observable of data in the database.

My application then has a local copy of data, in an array, that may have some of the same records, different records, etc. I am trying to combine the results into an internal BehaviorSubject that would then be used by my component to show the data.

    private ApiEndPoint: string = '/timeentry';
    private Data$_: BehaviorSubject<Array<LIB_TimeEntry_ResponseDTO>> = new BehaviorSubject<Array<LIB_TimeEntry_ResponseDTO>>(new Array<LIB_TimeEntry_ResponseDTO>());
    private MergedData$_: BehaviorSubject<Array<LIB_TimeEntry_ResponseDTO>> = new BehaviorSubject<Array<LIB_TimeEntry_ResponseDTO>>(new Array<LIB_TimeEntry_ResponseDTO>());

    constructor(
        private Http_: HttpClient,
    ) {}

    // component uses this with async pipe and can loop through and show data
    get Entries$(): Observable<Array<LIB_TimeEntry_ResponseDTO>> {
        return this.Data$_.asObservable();
    }

    // component shows this value using async pipe, which is correct, so the Entries$ does have the data from the backend
    get Count$(): Observable<number> {
        return this.Entries$.pipe(
            map( (Entries: Array<LIB_TimeEntry_ResponseDTO>) => {
                return Entries.length;
            })
        );
    }

    get MergedData$(): Observable<Array<LIB_TimeEntry_ResponseDTO>> {
        return this.MergedData$_.asObservable();
    }
    
    // component calls this to load intiial data when starting
    public Initialize(BaseURL: string, UserId: string, EntryDate: number): void {
        const AccessEndPoint: string = `user/${UserId}/${EntryDate}`;
        this.Subscription$_ = this.Http_.get<Array<LIB_TimeEntry_ResponseDTO>>(`${BaseURL}${this.ApiEndPoint}/${AccessEndPoint}`, { observe: 'response' }).pipe(
            catchError(() => {
                console.log('Error received in TimeSheet Service');
                return throwError(() => {
                    console.log('Error rethrown by TimeSheet Service');
                    return new Error(`Unable to load Time Sheet data`);
                })
            })
        ).subscribe(data => {
            this.Data$_.next(data.body as Array<LIB_TimeEntry_ResponseDTO>); // Loads into internal BehaviorSubject
        });
    }

    // Getting the data from the backend seems to work.  Now I am trying to loop through this data, and combine it with an array
    // of data I have into a new BehaviorSubject.  In my testing I call this as a public function from the component but it doesn't work
    public MergeDatabaseDataWithUploadedData_(LocalData: Array<LIB_TimeEntry_ResponseDTO>): void {
        const DatabaseData: Array<LIB_TimeEntry_ResponseDTO> = new Array<LIB_TimeEntry_ResponseDTO>(); // Create the array to build the data into
        
        this.Entries$.pipe( // Follow Count$() call to get the Array (Data) to work with
        map ( Data => {
            Data.forEach( (OneEntry: LIB_TimeEntry_ResponseDTO) => { // Loop through each record and add it to my Array for subsequent use.
                DatabaseData.push(OneEntry); // Data does not appear into my array. Trying things, as I did want this to be a Map, but array illustrates the point.
                // const KeyValue = this.BuildUniqueKey(OneEntry); 
                // Map.set(KeyValue, OneEntry);
            });
        })
        // ).subscribe(Data => { // HAd tried this thinking it was the problem?
        //  return Data;
        // })
        ;
        console.log(DatabaseData); // No Data???

        // At this point, I will process through my LocalData updating information
        LocalData.forEach( (OneRecord: LIB_TimeEntry_ResponseDTO) => {
            ... // Determine if record exists
            const LocalKey = this.BuildUniqueKey(OneRecord);
            if (Map.has(LocalKey)) {
                ... // Make changes to OneRecord
                Map.set(LocalKey, OneRecord); // Update Map entry from DB with Local Results
            } else {
                ... // Make OneRecord indicate this is a new record
                Map.set(LocalKey, OneRecord); 
            }
            ... // Additional logic to indicate records to be deleted
        });
        const MergedData: Array<LIB_TimeEntry_ResponseDTO> = new Array<LIB_TimeEntry_ResponseDTO>(); // Make array to process Map into
        Map.forEach( (OneMapRecord) => {
            MergedData.push(OneMapRecord);
        });
        this.MergedData$_.next(MergedData); // This would be the Map data that has been built
        
    }

I have more code trying to merge data etc., but as I cannot simply convert the data into an internal array, the remainder of the code will obviously not work until this first issue is resolved.

Once I can populate the Array(Map in the future) from the observable, I will then loop through my local array data, and update the records in the Array from the observable. This updated