buildscript section has to be compiled running your projects build scripts. So it does not know anything about your projects script elements. In example above you declare extra parameter in build script and retreive it in your project script, cause project script have access to parameters map in build script

how does this work tho

val kotlinVersion : String by extra

does it get property by the same name from extra?

val nameOfProperty: TypeOfProperty by extra

in build script you have to register your proprty , so you write val nameOfProperty by extra = valueOfProperty

no

plugins block live another live

their purpose to be as much determenistic as possible

for more explanation have a read this: https://docs.gradle.org/current/userguide/kotlin_dsl.html#type-safe-accessors