I would recommend to read the ordinals BIP to understand how it works:
https://github.com/ordinals/ord/blob/master/bip.mediawiki

And if you want the full tech information, you can see the docs on the main ordinals site:
https://docs.ordinals.com/introduction.html

And that site you will understand how to work with ordinals step by step, and the limits of the technology.

You mentioned the sites that make ordinal don't have their code open source, and that's because anyone can code it's own program with the current info in the docs.

And one last source of ordinal info is Bitcoin-Knowledge repo:
https://github.com/vondas-network/Bitcoin-knowledge#ordinals