I have an iPad 8 connected to a monitor via the Apple HDMI cable and 3rd party HDMI cable, its working great, but now I’m wondering if it’s possible to use the 2nd monitor not to mirror the display as is currently the case but rather to extend it.
Currently it isn't possible to extend an iPad's display. You can mirror it to another screen (via HDMI or AirPlay) but not use an external screen as a separate display. You can only use the iPad as an external display for a Mac.
This feature wouldn't make much sense unless it would be required for a keyboard and mouse/trackpad to be connected. In the end an iPad is only a tablet and it would be impossible to control anything on the second monitor without peripherals. Maybe it's still a possibility in the future since Apple likes to advertise the iPad as a notebook replacement but that remains to be seen.
This has already been answered in the Apple Community Forum in 2020. However, this hasn't changed since then: https://discussions.apple.com/thread/251065889