Many Developers only support one or two earlier major versions of iOS/iPadOS. If you cannot see and download an App from the App Store - that you have previously download/used with that device - then the earlier version of the App has likely been withdrawn by the respective App Developer.
Once App version is withdrawn, it is no longer supported or available. Other than the Apple App Store, there are no App archives from which an older App or version can be installed.
To check for an available App version, open the App Store App. At top-right, tap your initials to access your account. From the menu, select Apps. This will open a list of Apps previously downloaded/purchased; search the list for the App that you would like to download/reinstall. If a compatible download is available, you'll see a blue cloud icon with a downward-pointing arrow. Tap the icon to download.