You need to mark your assets as available to be Batch Exported. Go to each artboard, then select all the items that you want to export. Then right click, in the contextual menu there should be an option to Mark for Batch Export. You can also do this on the Artboard itself, but it will export the artboard as a single element. With your items marked for Batch Export, File > Export > Batch… will be available. In addition, once they have been marked, when you publish a Design spec, they become available through that system as well.
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