Forms submission

Depends on your use case as to what you'd like to do with those people. Does your use case need the list of people to be static or dynamically updated every time you refresh/reload it? With the static list, you'd need a smart campaign in place to continuously add people to it (if you wish to keep it updated). However, with the smart list, the qualifying people based on the set filters would automatically be pulled into it. Static lists are more suitable for use cases where you want a snapshot of people who qualify for certain filters at a given point in time, however, as mentioned above smart lists are dynamic in nature and are updated every time you reload it/referenced by other assets (nested smart list, campaign, etc.).

To give you an idea, below is the filter you should be using to pull the list of people who filled out form on a particular landing page. FYR, select the Marketo landing page in the Webpage constraint. However, if the form is placed on a non-Marketo page, you'd wanna use the Referrer constraint with the page URL instead.

You could have a static list and, and within your Program flow step add all form fills from the landing page in to the list.

A Smart list would give you the most flexibility on filtering and segmenting your form fills  - you can add filters for all time form fills, last month, based on Country/language or other segmentations of your choice.