Gotcha!
That is a template screen which MSFT provides. By default, it is now adding a Data Source (Office365Outlook) to your app, and then performing relatively complicated functions to organize and present the data to do.
For example, the OnSelect of the Calendar Drop Down is this function
/*retrieves calendar events for all days in current month view and selected calendar
_minDate and _maxDate act as markers to prevent duplicate data collection*/
If(IsBlank(_userDomain),
UpdateContext({_showLoading: true});
Set(_userDomain, Right(User().Email, Len(User().Email) - Find("@", User().Email)));
Set(_dateSelected, Today());
Set(_firstDayOfMonth, DateAdd(Today(), 1 - Day(Today()), Days));
Set(_firstDayInView, DateAdd(_firstDayOfMonth, -(Weekday(_firstDayOfMonth) - 2 + 1), Days));
Set(_lastDayOfMonth, DateAdd(DateAdd(_firstDayOfMonth, 1, Months), -1, Days))
);
Set(_calendarVisible, false);
UpdateContext({_showLoading: true});
Set(_myCalendar, dropdownCalendarSelection1.Selected);
Set(_minDate, DateAdd(_firstDayOfMonth, -(Weekday(_firstDayOfMonth) - 2 + 1), Days));
Set(_maxDate, DateAdd(DateAdd(_firstDayOfMonth, -(Weekday(_firstDayOfMonth) - 2 + 1), Days), 40, Days));
ClearCollect(MyCalendarEvents, Office365Outlook.GetEventsCalendarViewV2(_myCalendar.Name, Text(_minDate, UTC), Text(_maxDate, UTC)).value);
UpdateContext({_showLoading: false});
Set(_calendarVisible, true)
You can definitely tweak this function to meet your needs.
HOWEVER, you will need to also review all of the controls that are added to the screen and tweak them to meet your needs. This is not a simple undertaking; but gives you a great place to start.
This guide will help you: https://docs.microsoft.com/en-us/powerapps/maker/canvas-apps/screen-templates/calendar-screen-overview
