Share survey—Survey123 for ArcGIS | ArcGIS describes how to share a survey to everyone once you've authored a survey. There's nothing particularly special about a public survey in the authoring process, though you may want to think about the portion of the public you want/expect to fill in the survey- what is their language fluency breakdown (English, Spanish, Korean, etc.), level of sophistication (e.g., if reporting wildlife sighings, do they know the difference between a raven and a crow).
In terms of security - surveys currently published by Survey123 Connect by default are published with full editing controls enabled (add/update/delete), though it takes some technical knowledge to access those capabilities (we don't provide editing controls in the user interface, just data submission). This can be set to add only in the ArcGIS Online/ArcGIS Enterprise settings for the Feature Layer (data table) associated with the survey. Surveys published through he Survey123 website's form designer expose only a 'fieldworker view' of the survey to the public; when published, the default for this is add data only.
