Data transfer

See:

Transfer data from your previous iOS or iPadOS device to your new iPhone or iPad - Apple Support

Although it isn't stated in the linked article above whether or not data is deleted from the source device or not, in my personal experience doing this on multiple devices in the past, the data is NOT deleted from the source device automatically as it should be (in case something went wrong in the data transfer).

See the link below for instructions on how to erase the source iPad after it has been verified that ALL data was successfully transferred over to the destination device.

Erase iPad - Apple Support

“Transfer” is rather a misnomer. In reality, your data and setup is merely cloned to the new device. Nothing is moved or removed from the original device.

When you are ready, the best procedure for preparing the old device for disposal or reuse can be found here:

What to do before you sell, give away or trade in your iPhone, iPad or iPod touch – Apple Support

All the files do stay on the old iPad.

If your transfer is successful and after verifying all your files work as expected on the new device, you can manually remove the data file by file or erase the old iPad entirely.

If you intend to sell the old iPad then it's best to erase it and remove the Activation Lock (your AppleID) from it.