How can I get an older version of an app for my iPad and iPhone?

If you own much newer iOS device that can run either iOS 12 and.or iOS/iPadOS 13 and later, you can install the most recent app/s onto that more recent iOS device.

Then go back to your older iPad, open the iOS App Store, go to the Purchases section of the store app, search for the app/s you wish to try and download to that older iPad and see if an older version of the app is allowed to be downloaded/installed to that older iPad.

These newer iOS/iPadOS devices must use your same Apple ID user account and password as the older device you ard trying to get an older app for.

Failing that OR IF NO joy, then there is nothing more to be done.

if you own an 8-10 year old OR older iOS devices, Apple ended support for all 8-10-year old iOS devices well over 4 years, ago, in 2016.

Third party app developers abandoned and stopped supporting these 8-10-year old iOS devices nearly 3 years, ago, in 2018.

There are, virtually, NO MORE third party apps available from the iOS App Store, any longer!

Third party compatible apps for such old devices are few and far between, now, in 2021.

This includes apps for both iOS 9 AND iOS 10 devices!

Best of Luck to You!

Even if you can download an earlier version, you may come across the issue that it will not be compatible with the version of iOS or iPadOS, running on your devices.

Regardless, you still may have two options:

1. Since, most likely, they will not be available at the Apple App Store, you can try downloading them from the app developer's site.
2. From a recent backup of these devices where you had this app installed. Again, this may not work if you upgraded the OS on these devices.