These are third-party apps that you can install on your iPhone to sign and install IPA files directly, without a computer. They are popular for their ease of use, but they are also the most unstable. They rely on enterprise or developer certificates to function. When Apple discovers these certificates are being used for widespread distribution of cracked apps, they them. This causes the app to crash immediately on launch. Users can then look for a new, un-revoked certificate or a new version of the app. Despite the name "Scarlet" being advertised as "never revoking," the reality is that no third-party signing service is truly immune to Apple's enforcement.
Some users in countries with limited Netflix libraries turn to modified apps hoping to bypass geo-restrictions and access content from other regions. netflix premium ipa file install