Starting with AirPlay, this feature uses your WiFi connection to send video and sound to a compatible AirPlay receiver - such as an AppleTV box, or a Smart TV that supports AirPlay. When this connection is active, you’ll see an icon at top-right of your screen.
More information about AirPlay - and how to use it - can be found here:
Use AirPlay to stream video or mirror the screen of your iPhone or iPad - Apple Support
Other methods of connecting to an external computer monitor or TV will require a wired connection - of which there are several types of connection. For iPad9, you’ll need an appropriate adapter for the connection type that you wish to use - and a matching cable - the type of connection principally being determined by the computer monitor or TV to which you are attempting to make connection. The iPad9 supports HDMI and VGA video connections.
For HDMI, you will require an Apple Lightning Digital AV Adapter:
https://store.apple.com/xc/product/MD826ZM/A
For VGA, you will instead require an Apple Lightning to VGA Adapter:
https://store.apple.com/xc/product/MD825ZM/A
You can find more information about connecting your iPad to external monitors within the [frequently overlooked] iPad User Guide - this being a rich source of information about your iPad and its many features and capabilities. Here is a direct link to the relevant section of the Guide:
Connect iPad to a display with a cable – Apple Support
For information, the full User Guide is available both online using a web browser (such as Safari), or as an Apple Books download:
iPad User Guide - Apple Support
https://books.apple.com/book/id1567104892
The Tips App, installed by default on all new models of iPad, provides a brief introduction - and leads you to the User Guide. In case you missed it, here is a link to the Tips App:
https://apps.apple.com/gb/app/tips/id1069509450
Apple also has a huge YouTube Channel - containing videos and feature guidance:
https://www.youtube.com/user/Apple
