Exclamation [Q] Building a recovery.img from scratch

Here is the theory I want.
Given a mobile phone that has a lock on it wether pin code, pattern or password, If the mobile's bootloader was unlocked I can install a custom recovery and then boot to that recovery in which I can run the adb and have access to the data in the phone. If my hypothesis is correct then how I can build a recovery.img ??

I have been reading for the recover.img for 2 weeks and almost understood how it works and most of the files in it. However, I couldn't find a tutorial of how can I build my own recovery.img

What I decided to do it to modify on the CWM recovery.img.
So I downloaded the CWM and then I unpack it to see the kernel and the initrd. After decompressing the initrd, I found many files. I made a simple test which is to run a script from the init.rc. Inside the init.rc I added two lines. The first one to mount the /data partition and then I just echoed "test" to a file called text.txt then I rebooted my device to normal boot connected to adb and checked the /data partition, but unfortunately I didn't find anything.

I would like to know what I am missing and if my theory is correct or not and how can I build my OWN recovery and have something like adb running from the start.

Thank you