External display opens the same app as the main screen on my MacBook

There are two distinct ways of using those two displays.

The one you are using now is a variant of Extended desktop, but you have checked [√] Mirror displays, so you get the same picture -- not the same Apps, EXACTLY the same picture on each display. You can connect the displays into one extended desktop by unchecking that box:

The displays are concatenated along an edge into one big Extended Desktop, and once you arrange them is space, the mouse moves freely across the boundary between displays, and can carry a winew with it. You can drop the window on the break between displays, and it will continue to scroll "the right way"even though half is on each display.

The other way of working is to treat each display as a completely separate space, with NO Overlap between displays., You move you attention from one to the next. This is an extensions of Mission Control "Spaces" that can be swapped in and out of the current display. It is simply enhanced by having different "displays have separate spaces".

If you want to use "displays have separate spaces", the conventional way to do that is to assign each Application to its own Space.

Then when you click on Excel, the space that contains Excel will open, showing one or more windows in one space on ONE display. When you click on Word, the Space that contains Word will open.

See if this glossy article is any help:

https://appleinsider.com/articles/18/10/12/how-to-use-spaces-apples-mostly-ignored-macos-mojave-productivity-feature

or maybe this one:

https://www.howtogeek.com/180677/mission-control-101-how-to-use-multiple-desktops-on-a-mac/

It is not an accident that one of the commands inside Mission Control is "Group Windows by Application". That is what they expect you to do.

Change Mission Control preferences on Mac - Apple Support

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