Where can I get specification of COG?

[kochizufan]

I'm searching pure-JavaScript/TypeScript (Node.js) solution to change WMTS tile images(Jpeg/Png) into COG.
I can't find such solution, so I have to develop it by myself.

But I can't find any information of GeoTIFF image specifications which includes data mapping of image binary.
What I looked were:
https://github.com/cogeotiff/cog-spec/blob/master/spec.md
https://docs.ogc.org/per/21-025.html
But these pages not include data structure itself.

How can I get basic data structure information of COG?