Dataflows are a feature within Power BI which is used to connect and transform data to utilize them as datasets when designing reports. It is a good way to create reusable datasets that are refreshed each time when a change occurs. They can be used by different users within a company to create reports not only in Power BI, but also in other Power Platform services.
Power BI Desktop reports are stored in PBIX files that contain connections, datasets, transformations, visualizations, and other report elements. If you want to save reports without data, you will use the option of creating a template, by saving as a PBIT file. If you want to save only the link to the data source you will create a PBIDS file.
Pivot tables are reports that, above all, are used to interpret data based on given criteria. Sometimes a pivot table can also be used as a data source to make one or more new pivot tables. This usually happens when you want to create a pivot table from an existing business report, based on a query to the transaction database, and thus use the data that are available in initial pivot table.