MyBB Community Forums

it was suggested I look here https://docs.mybb.com/ how ever I cant find anything for custom permissions on who can see what forums/categories and what not

Though the document is for v1.6 and outdated but still mostly valid:
https://docs.mybb.com/1.6/Admin-CP-Admin-Perms/

If the available options are not covering your requirement and you are looking for some custom user permissions you can use this plugin:
https://community.mybb.com/mods.php?action=view&pid=807