Basically, “Build Artifact” is a conditional build, meaning it will only build if there is any modified code. If you haven’t changed anything, it won’t do anything.
“Rebuild Artifact”, however, will force a build from scratch no matter if there are code changes or not.
From the horse’s mouth: https://intellij-support.jetbrains.com/hc/en-us/community/posts/206866915-understanding-build-vs-rebuild-vs-make-vs-compile