Every campaign and every flow email has its own design, but the Templates page only lists designs that have Save as Template switched on (tooltip: When enabled, this design will appear in your Templates list for reuse in future campaigns.). Toggle it in the designer's top bar; a message Saved as template / Removed from templates confirms the change immediately. Designs created from a campaign or flow start with the toggle off, which is why they are not in the list.

Only saved templates are offered in the campaign wizard's Start from scratch or choose a template picker. Removing a template from the list (the trash icon or the bulk Remove from templates action) simply switches this toggle off – The design will no longer appear as a reusable template but won't be deleted, so campaigns using it are unaffected.