Persist data with Room -- Variable without Instruction

[alarimer]

URL of codelab:
https://developer.android.com/codelabs/basic-android-kotlin-compose-persisting-data-room?continue=https%3A%2F%2Fdeveloper.android.com%2Fcourses%2Fpathways%2Fandroid-basics-compose-unit-6-pathway-2%23codelab-https%3A%2F%2Fdeveloper.android.com%2Fcodelabs%2Fbasic-android-kotlin-compose-persisting-data-room#3

In which task and step of the codelab can this issue be found?
4. Persist data with Room

Describe the problem
Adding the code to include the Room dependencies references the variable stored in ${rootProject.extra["room_version"]} without mention of that already existing, how to implement it in a project from scratch, why this pattern is utilized, etc.