Without a jailbreak, you use a signing service.
The last official version to support iOS 9.3.5 was roughly YouTube v15.49.6. Repacks of this version are the most stable. youtube ipa for ios 935 repack
You are downloading a binary file from a stranger on the internet that has the ability to fully connect to the internet, manage files, and (if jailbroken) take over your device. Without a jailbreak, you use a signing service