Importers in HMCI are classes that extend the HMCI\Iterator\Base class. This class provides a standard interface for iterating over data, and a standard interface for processing data. An importer is typically a single data type, so it's likely that a project migration will have multiple importers.
Think of an importer as a "data source" which will fetch items to be imported, and then process each iteam to be imported. An importer need only implement the get_items( int $offset, int $count ): array<mixed> method, which is used to fetch items from the data source. The type of each item returned by this method is up to the importer. Optionally get_count(): int can be implemented to return the total number of items in the data source, if the count can be calculated in a more performant way than count( while ( get_items() )... ).
The importer class then must implement the process_item( mixed $item ) : bool method, which is responsible for importing the item. This method is passed each item returned by the get_items() method. The bulk of the importer logic is implemented in this method.
As mentioned, all importers should extend the HMCI\Iterator\Base class. That is because importers are Iterators. It's unlikely you'll extend the HMCI\Iterator\Base class directly though, as HMCI providers a number of base iterators that you can extend to create your own iterators. For example, if you are importing data from a collection of CSV files, you can extend the HMCI\Iterator\Files\CSV class to create your own iterator. This way you will only need to implement the process_item( mixed $item ) : bool method, and HMCI will handle the rest.
See all the iterators in the inc/classes/iterator folder.
Inserters in HMCI are classes that extend the HMCI\Inserter\Base class. HMCI bundles standard inserters for most WordPress types, but it's possible to write your own inserters for custom data types.
The Inserter class will automatically handle the insert/update logic based off a "canonical id" field (see below).
See all the importers in the inc/classes/inserter and the WordPress specific inserters in the inc/classes/inserter/wp folder.
When writing importers with HMCI, all items that are being imported will have a canonical ID. This is a unique identifier from the source data, and is used to match already imported items in the WordPress database.
Note: canonical IDs should be unique accross importers if those importers are importing the same WordPress data type. For example, if you have an importer that imports articles with canonincal IDs ranging from 1-100, and another importer imports products that also have canonical IDs ranging from 1-100, then the two importers will have a conflict. Therefor it's recommended to use a prefix for the canonical IDs, such as article_1, article_2, etc.
Check the examples below to see how to write your own importers: