This is a short guide on how to make the auto arm grab only one item in Astroneer. Believe it or not I get asked this question maybe once a week.
The auto arm will, by default, grab any item or resource that can fit into a single slot. This is by design.
You can filter what the auto arm grabs by placing an item or resource into the top slot of the auto arm, as shown below.
Once you insert an item, the auto arm will grab only that item. Refer to the video guide embedded at the top of this article if you need more help!