Is https://docs.gradle.org/current/dsl/org.gradle.api.artifacts.Configuration.html#org.gradle.api.artifacts.Configuration:extendsFrom "live", i.e. if B extends from A, and I modify A afterwards, are those changes reflected into B?

Of course, everything else would be extremely strange. Typically a plugin (like the

java

plugin) sets up these constructs (like

runtimeClasspath.extendsFrom(implementation)

) loong before the dependencies are added.