Most modern apps require iOS 13, 14, or higher. Unless the developer has kept an older version of their app on the server, the App Store will not let you download it. This is where manual IPA files come in: they allow you to install older versions of apps that are no longer available through the official store.
: Once jailbroken, use IPA Installer from Cydia to browse your device's storage and install files directly. Method C: Modern Sideloaders ipa files for ios 7.1.2