Visit a reputable APK repository like APKMirror or APKPure .
If you are on an iPhone 13 or newer running iOS 16+, you cannot get a truly old native app without jailbreaking. Your best bet is Android or desktop. how to get old version of youtube
You cannot technically "downgrade" the website itself, but you can use scripts and extensions to change the look and functionality. Visit a reputable APK repository like APKMirror or APKPure