get_site_transient() uses the older nomenclature for multisite which referred to a multisite network as a “site” and individual sites on the network as “blogs”. So get_site_transient() is getting the value of a transient for the whole network, while get_transient() gets a transient for an individual site/blog.
If you look at the source of the function you’ll see that it uses get_site_option() internally, and the documentation for that function reads (emphasis mine):
Retrieve an option value for the current network based on name of
option.
Also note that get_site_option() has been effectively replaced with get_network_option(), but for some reason the same change hasn’t been made for the transient function.