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Cydia was removed from the Procursus bootstrap (the modern jailbreak standard) because it is incompatible with rootless iOS 15. However, you can force-install it using the cydia.app deb file from BigBoss, but you must:
You can use the Palera1n jailbreak. It is semi-tethered, meaning you need a computer to reboot into a jailbroken state. cydia download ios 15.7.1
Now, to answer the core keyword: