Hi @Al_Scotch,
Thanks for your questions.
FeatureFromDescription is intended to be used from within the context of GotFeatures event. The second parameter to that event, features is a list of feature descriptions parsed from GeoJSON. You can iterate over each element in this list to (optionally) manipulate or filter it before passing it to FeatureFromDescription. The elements of features are associative lists containing the key-value pairs from the GeoJSON for the given feature. (Maps predates the addition of dictionaries to the App Inventor language, otherwise they would have been dictionaries).
Note: If you don't provide an implementation for GotFeatures (i.e., you don't include the event block in your project), then the system will provide a default implementation that will pass each feature description directly to the method add the resulting component to the Map or FeatureCollection.
For CreateMarker, it creates a instance of Marker, adds it to the Map, and returns the marker, which you can pass to the "Any Marker" blocks, if further customization of the marker is desired. If you don't need to customize the marker, you can connect this block to the input of the evaluate but ignore result block.