It seems you're looking for the YouTube app compatible with (the latest version available for older devices like iPhone 5s, iPhone 6, iPad Air, etc.).
Here's the text you can use or search with: "Download YouTube app for iOS 12.5.7 – compatible version for older iPhones and iPads" Option 2 – Descriptive text for a guide or post: "Looking for the YouTube app on iOS 12.5.7? The latest compatible version is YouTube 17.39.4 (or newer, if still supported). You can download it directly from the App Store. If you see 'This app requires iOS 13 or later,' tap 'Download Last Compatible Version' when prompted. No jailbreak needed – works on iPhone 5s, 6, iPad mini 2, iPad Air, and iPod touch (6th gen)." Option 3 – Simple copy-paste for a search engine or store: "YouTube for iOS 12.5.7 download – App Store last compatible version" youtube app for ios 12.5 7 download