Please keep in mind that different Android phones run different flavours of Android, sometimes highly costumized by the vendors. Rooting procedures can and will be different between phones, as will be risks and consequences going along with it. (Even between Fairphone 2 and Fairphone 3 there are differences.)
You may think of it as different Linux distributions: Debian, Fedora, Slackware, Gentoo, Arch Linux all have their own way of managing details, configurations, etc. There’s lots of distribution-specific documentation.
If you wanna master one of those distributions on a system administrator’s level (which is what root is all about), you need to dive into the specific flavour accordingly.