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init is not "spawned" (as a child process), but rather exec'd like this:

# Boot the real thing.
exec switch_root /mnt/root /sbin/init

exec replaces the entire process in place. The final init is still the first process (pid 1), even though it was preceded with those in the Initramfs.

The Initramfs /init, which is a Busybox shell script with pid 1, execs to Busybox switch_root (so now switch_root is pid 1); this program does some magic with the mount points so /mnt/root will be the new /.

switch_root then again execs to /sbin/init of your real root filesystem; thereby it makes your real init system the first process with pid 1, which in turn may spawn any number of child processes.

Certainly it could just as well be done with a Python script, if you somehow managed to bake Python into your Initramfs.

However, it does not work on kernels that do not allow script binaries (CONFIG_BINFMT_SCRIPT=y), or rather in such a case you'd have to start the interpreter directly and make it load your script somehow.