You can easily reuse your report and data source files for Data Studio by properly copying them. Below are the resources from Google that show step-by-step how to do each.
Please note that the report file and the data source file are separate objects in Data Studio. If you want to reuse your custom calculated fields, they need to be defined in the data source file itself and you need to copy the data source file.
Copying Data Studio Reports
See https://support.google.com/datastudio/answer/7175478 for how to properly copy the report. This is useful if you want to build one report as template and then duplicate it for other brands/clients!
Please note that some elements are part of the data source file, not the report, and won't be copied over unless you also recreate/copy the data source file.
Copying Data Source Files
See https://support.google.com/datastudio/answer/7421646 for how to properly copy data source files. This is useful if you want to reuse complex calculated fields you created without having to make them from scratch every time!
Note that this copies the user credentials used for the Supermetrics connector so if you don't have access through that, the copy won't work properly. It also does not copy chart-specific calculated fields and data blended sources.