If you accidentally restart the ipad while updating, will it resume downloading and updating the ipadOS or not?
It should, but if it doesn't just go back to Settings > General > Software Update and start the update over.
It should, but if it doesn't just go back to Settings > General > Software Update and start the update over.
The state of the iPad at the point of interruption will heavily influence what will happen. If the initial software download is interrupted, then you iPad may not suffer any specific consequences. However, if the downloaded firmware is being actively “flashed”, the operating system may subsequently fail to boot - potentially requiring a DFU Restore to recover.
Assuming that this is a hypothetical question, until a explicit issue is encountered which will determine an appropriate recovery action, a definitive response to your question is difficult.
Software Update
