Due to certificate revokes by Apple and the age of the firmware, many modern libraries do not work. Here are the ones historically compatible with iOS 9.3.5:

Click the Start button. Enter your Apple ID password if prompted (using an App-Specific Password is recommended for security).

Use legacy utility tools such as PhPhoenix (semi-tethered) or p0weruser to jailbreak iOS 9.3.5.

Don't underestimate the power of the Internet Archive. This massive digital library isn't dedicated to iOS apps, but it is a goldmine for finding old, obscure, and abandoned IPA files. Because many developers and users have uploaded their collections here over the years, it's an excellent place to search for a specific version of an app that you can't find anywhere else. The main drawback is that its interface isn't tailored for iOS app discovery, so it can take some searching to find what you need.

Downloading apps from unverified third-party sources requires careful attention to safety protocols.

Enter your Apple ID and password (required by Apple to sign the app).

Establishing an IPA Library for iOS 9.3.5 is a common pursuit for enthusiasts of "legacy" Apple hardware, such as the iPad mini 1