How can I switch desktops on built-in display and external monitor independently?

Hello!

I have been using an external monitor (E20 display) with my MacBook Air (13-inch, early 2015; macOS Catalina, 10.15.7) for the past several years. When I first used it, I could switch between different desktops on either the main (built-in) screen or the external monitor independently of each other. Specifically, if my pointer was on the main screen, and I moved to a different desktop, the desktop on the external monitor would stay the same and the one on my main screen would change (and vice versa--if my pointer was on the monitor screen and I swiped to switch desktops, the desktop on the main screen wouldn't change). Moreover, when I wanted to move a desktop from one screen to the other, I could press f5 and drag the desktop to the desired screen.

At some point, I was unable to do this anymore. The main screen and monitor no longer act independently of each other--when I swipe to switch to a different desktop, whether my pointer is in the main screen or the monitor, the desktop changes on both screens.

I still connected the computer to the monitor the same way and I did not change the display options. I tried searching online for a way to change the display settings back to the way they used to be but was unsuccessful.

I apologize if I am not using the proper terminology! I hope you understand what I am trying to say.

I would appreciate any input you have on this issue! Thank you so much ☺