Hi dmtri2012,
To make sure we're on the same page, it sounds like you need access to a file in the Files app that was stored locally on your iPhone's internal storage, is that correct?
The files and folders that you add to the Files app on an iOS device either store on the device locally or sync to iCloud: Use the Files app on your iPhone, iPad, or iPod touch. If the files were syncing to iCloud, they would not be included in an iCloud backup because that selected data is set to sync to iCloud already: What does iCloud back up? Files synced to iCloud in the Files app can only be accessed from a device that is signed in with the Apple ID they are synced with in iCloud, too: Use iCloud.com
Also, if the files were being stored locally on your iPhone and you performed a backup on a computer at some point to save them, you still do not have the ability to selectively view or restore specific files only from within a backup. You would need to restore the entire backup to another iOS or iPadOS device in order to regain access to that data: Restore your iPhone, iPad, or iPod touch from a backup
Let us know if you have more questions after reading these details. Or, if we misunderstood where you were saving these files, feel free to reply back with more details.
Take care.
