Data from external systems can be provided to Autotab in the form of CSV files or string inputs, which can be passed to the API to parametrize skills. However, in most cases, ingesting data into Autotab is easiest by just having Autotab navigate to the website where the data is present.
Autotab has a structured type system underlying the workflows, so any data processed in the course of an automation can be referenced in later steps. It's a bit like a fuzzy programming language for automation, and the model generates schemas to ensure data flows reliably through the series of steps.
For example, users often start by collecting information in one system (using an extract step as you mentioned), then cross reference it in another and then submit some data by having Autotab type it into a third system. In Autotab, you can just type @ to reference a variable, each step has access to data from previous steps.
At the end, you can get a dump of all of Autotab's data from a run as a JSON file, or turn specific arrays of data into CSV files using a table step.