Yeah the menus and the armor perks can be very difficult to learn intuitively what they do. Back when the game was newer there were several you tubers who tested everything and made a lot of videos about it.
PWAR gaming was a popular channel:
https://www.youtube.com/channel/UCK3pISmU5NW9wZliwNqBN1Q
He tested out all the perks and made a lot of video's of OP builds that break the game. Although it sometimes felt like Team Ninja was watching his channel and constantly nerfing his builds, there were other people using the same builds. Keep in mind that his OP LW build got nerfed really hard.
A lot of the info on how the game works and how the menu's work is buried in youtube videos of several you tubers who played the game for 1000s of hours. People didn't generally get around to writing a lot of long guides.
For example I learned how to make the OP level 999 abyss one shot build by watching youtube videos and pausing the video's and taking screen shots to look at the perk combinations when they were slowly flashed across the screen and talking to a few hardcore players on the Nioh reddit discord server. And that was back when the game was more popular. I made a post back in this thread about how to make a level 999 abyss one shot build. This build was nerfed but it can still one shot every boss in the Abyss even floor 999.
In general if you do a search on YouTube there tends to be a decent video explaining just about anything you want to know about Nioh including the Menus. Unfortunately it wasn't really written into a centralized Wiki's or text based guide. Although I think it is true that the knowledge of how every system in Nioh works is fully known ... but on Youtube in various scattered videos.
If you want to read then this is a good thread to get started:
https://www.reddit.com/r/Nioh/comments/6u8x4y/the_kodamas_directories_your_onestop_thread_for/