You can place your script into /etc/init.d
root@imx8qmmek:/etc/init.d# cat test.sh
#!/bin/sh
echo test > /file.txt
root@imx8qmmek:/etc/init.d# ls -l test.sh
-rwxr-xr-x 1 root root 22 May 25 18:50 test.sh
root@imx8qmmek:/etc/init.d#
and add it into exact runlevel
root@imx8qmmek:/etc/init.d# runlevel
N 3
root@imx8qmmek:/etc/init.d# cd /etc/rc3.d/
root@imx8qmmek:/etc/rc3.d# ln -s ../init.d/test.sh S50test.sh
root@imx8qmmek:/etc/rc3.d#