An individual's server-wide permissions are determined by adding up all the allows for roles assigned to that individual along with the allows for @everyone.
Channel Permissions
Channel permissions start with server permissions as a base. Then, the hierarchy is as follows:
Apply denies of @everyone on channel
Apply allows of @everyone on channel
Sum up all the denies of a member's roles and apply them at once
Sum up all the allows of a member's roles and apply them at once