While Terraform the tool can be used across cloud providers, Terraform configurations cannot.
Each terraform file uses modules that are quite specific to the individual services provided by a given cloud. These cannot be simply swapped out without rewriting the config.
In general, that's something that should be known in advance when someone chooses a cloud provider. As I mentioned, there is no one-site-fits-all solution. :/ Vendor lock-in is a serious issue for some enterprise companies, and in such cases you could propose something like a hybrid cloud. It's an expensive effort that could save your butt in the future.
Each terraform file uses modules that are quite specific to the individual services provided by a given cloud. These cannot be simply swapped out without rewriting the config.