You start it inside HEXOstudios. For 14 days you get every Pro feature at no charge, and nothing is purchased automatically when it ends. You can buy Pro during the trial or after it.
If you don’t, HEXOstudios returns to Free: your files, folders and organization stay exactly as they are, Pro features switch off, and your 12 + 12 free 3MF and STEP previews carry on.