Hi @David Ash,
Welcome to Affinity Forums
If you want to export your files as JPG's go to menu File > Export, select the JPG tab/format on top, set the options as you wish and press the Export button.
If you want to export/convert multiple afphoto files at once as JPG's you use File > New Batch Job... instead, click the Add button on the left to select the afphoto files you want to convert, set the appropriate options on the right and click OK.