You mp4 videos can be copied to your iPad from external storage just like any other file.
Your iPad9 has a Lightning connector - and as such will require an appropriate Adapter to connect your iPad to external USB storage devices. For your iPad, you will require an Apple Lightning to USB 3 Camera Adapter:
https://store.apple.com/xc/product/MK0W2ZM/A
Don’t be confused by the name of this Adapter - as it is the key to connecting many USB devices to your iPad. The adapter incorporates the essential USB Controller/Root Hub that interfaces USB accessories to your iPad’s Lightning port.
More information can be found in the iPad User Guide - along with other frequently overlooked information about your iPad and iPadOS:
Connect external storage devices to iPad - Apple Support
To be recognised by iPadOS, the attached USB storage device must be formatted to FAT, FAT32, exFAT, APFS or HFS+ with a single partition. Microsoft NTFS and other proprietary device formatting are not supported. Formatting and partitioning, if required, must be performed using a separate computer; iPad does not provide any facilities to format an external storage device.
As the iPad has limited power capabilities, dependent upon the USB Storage Device, an external power source is often required. Simply connect your iPad's Power Adapter to the Lightning port of the Adapter. Your Power Adapter must be rated at 12W or greater to provide sufficient power.
Having copied your mp4 (or other files) to your iPad, the can be accessed using the native Files App - or other media player Apps (such as VLC).