Here's how you can use the trackpad and a mouse with your Mac at the same time. This guide is only applicable to MacBook users.
macOS Allows You to Use the Trackpad and a Connected Mouse at the Same Time with Your MacBook
Using an external mouse with a notebook is very liberating. But on a Mac, the mouse routine has an interesting twist in it. When you connect an external mouse, you'll notice that the built-in trackpad stops working. That's fine with a lot of people as they already have an external mouse connected for all the pointing needs. But what if you need that trackpad and need to switch between devices very quickly? Well, it turns out, there's a simply toggle switch in macOS, which once unchecked, allows you to use the built-in trackpad and an external mouse at the same time.
This tutorial is only applicable to MacBook users only. If you have a Mac desktop with a connected trackpad and a mouse, things will work just fine. MacBook, MacBook Pro and MacBook Air users can follow the steps below to use the mouse and trackpad side-by-side.
1. Go to System Preferences by clicking on the Apple logo button in the menu bar.
2. Click on Accessibility in the list of available options.
3. In the left hand-side pane, scroll down and click on Mouse & Trackpad.
4. Now uncheck the last "Ignore the built-in trackpad..." option as shown in the screenshot below.
Now, keep one thing in mind: the option is disabled by default if you don't have anything connected to your MacBook. But once you connect an external pointing source, you will have to disable the option in the last step to use everything in perfect harmony.
Personally, I leave the option disabled because it helps with scrolling in my workflow a lot as you don't have to jump to the external mouse if you are just planning to do a slight scroll to bring something into view. Furthermore, the built-in trackpad offers a ton of gestures out of the box, something which an external mouse just can't pull off.