Need help with having a Monitor be the primary display with full functions, etc. vs. Macbook Pro being mirrored.

I want to have my Samsung monitor, connected to my new MacBook Pro be the primary monitor with full “functionality” using a mouse, showing full desktop, dock. etc. I used to have Windows computers at work and was able to do this. I would like to have two applications “running” at the same time, such as Excel on the large monitor and a separate application on the MacBook display.

I have turned off mirroring but then the monitor does not act as a primary.

Hope this makes sense. And thanks in advance for any help you can provide.