When does a package activation/instantiation trigger a build?

When is a build triggered for a package that has a deps/build.jl?
It is not triggered when I do activate/instantiate. Is there an automatic build?
Or is an explicit build() required?

https://julialang.github.io/Pkg.jl/dev/managing-packages/#Building-packages-1

The build step of a package is automatically run when a package is first installed. The output of the build process is directed to a file. To explicitly run the build step for a package, the build command is used:

That is correct information for a “local” install. But on mybinder.org the mechanism appears to be different.
I had to use a special script: Mechanism for build() in mybinder?

Perhaps the process of opening a package on mybinder is not really a “package installation”?
That would explain why build is not run. Could be that it is more like activating a local repository without
it being added as a package.