By default, preferences created by Preferences.jl are saved next to the Project.toml file of this package.
To my understanding, on an update of a package Foo, a new folder is created - e.g. the old version was in .../packages/Foo/4iQz5/ , whereas the new one will be put into .../packages/Foo/hJ0Fz/ .
Does that mean the preferences created under the old version of the package are not available under the updated version?
No, preferences get stored in your current active project (such as ~/.julia/environments/v1.11/ or wherever) not in the installed package’s project, and therefore persist across package upgrades.
Do I understand it correctly - if there is MyProject.jl, which is using package Foo, and that package Foo saves it’s preferences - those will be saved next to Project.toml of MyProject.jl?