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, `exec`s 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 `/` (and the old `/` vanishes into thin air, although you could use `pivot_root` instead if you had any reason to keep it).

`switch_root` then again `exec`s 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.