You add a piece of code (to ram, which famously does not hold information while unpowered).
Which scans for a specific very big prime number (finding large primes quickly would completely invalidate the world’s cryptography and therefore banking, that’s why people are afraid of the quantum boogeyman).
You look for any process and inject into stdlibc any backdoor of your choice (just any process, doesn’t need elevated permissions, assuming they use libc, assuming the backdoor hasn’t been patched out from the other end, defeated by any of the dozens of software integrity checks that have become standard).