You can preview the extensions by going to the form or portal URL. You can also click Open Extension (portal or shared view) / Share Form (form).
If the Form has no login, click Share Form, then Form for Creating Records to preview creating a new record.
To preview editing a record, click Share Form, then Form for Editing Records. A modal will be displayed. Click Try it out to open a random record or copy and paste the formula, then add it to a new formula field to your table to edit a particular record.
While you're logged in, you can edit your extension while accessing the form or portal. Just click Edit Extension in the bottom left corner. This button is not available on the end user's view.
On the free plan, you're only allowed to preview the extensions yourself. The extension would look and work to your users how it appears to you when you access its URL while logged in.