Your module’s go.mod file records which versions of dependencies it requires. The source code for those dependencies is stored in a local cache.
go get updates the requirements listed in your go.mod file. It also ensures that those requirements are self-consistent, and adds new requirements as needed so that every package imported by the packages you named on the command line is provided by some module in your requirements.
As a side-effect of updating and adding requirements, go get also downloads the modules containing the named packages (and their dependencies) to the local module cache.
In contrast, go mod download does not add new requirements or update existing requirements. (At most, it will ensure that the existing requirements are self-consistent, which can occur if you have hand-edited the go.mod file.) It only downloads either the specific module versions you’ve requested (if you requested specific versions), or the versions of modules that appear in your requirements.