GoKawiilSidecar is the simplest route, but there are a few requirements you need to meet. The most important one is having a Mac. Windows users are out of luck here, although there are alternatives in that case. If you already have a Mac, you're ready to go. From there, both devices need to be on the same iCloud account. It won't work if you use different accounts on your devices, so check your iPad Settings before continuing.
If your iPad and Mac are already signed in to the same account, using Sidecar is easy. On your Mac, open Control Center — it's the icon with two toggle switches in the Menu Bar. Look for Screen Mirroring, the two overlapping squares, in the list. Choose your iPad, and it turns into a secondary display for your Mac. From there it acts like any external display with full resolution and adjustable placement. You can even go to the Display settings under System Settings to rearrange the iPad's screen however you like.
iPad and Mac users also have access to Universal Control. Instead of turning the iPad into an external display, the idea is to use inputs from one device to control the other. Move the Mac's cursor onto the iPad's screen, send input the other way or even drag and drop between the two devices. Like Sidecar, both devices must be on the same iCloud account. Two-factor authentication has to be on, too. It works with pretty much every Mac and iPad released in the last few years.
If that doesn't work for you for some reason, try manually enabling Universal Control in the Screen Mirroring settings in Control Center. Look for Display Settings. Click "+" and select your iPad from the Link Keyboard and Mouse list.