In openEO, your process graph will always start loading some data, which can be done mainly using the following processes:
load_collection: loads a collection from the current back-end by its id and returns it as a processable data cube. The data that is added to the data cube can be restricted with the parameters spatial_extent, temporal_extent, bands and properties.load_stac: loads data from a static STAC catalog or a STAC API Collection and returns the data as a processable data cube. A batch job result can be loaded by providing a reference to it. If supported by the underlying metadata and file format, the data that is added to the data cube can be restricted with the parameters spatial_extent, temporal_extent and bands.
Learn how to load data with this interactive exercise: