If you create a base that other people can copy to their own workspaces and use on their own (within the limits of the free plan), then there is no cost.
If you create a web app using the Standard API, with your own method of validating users that doesn’t involve Airtable accounts, then there is no cost.
If you create an app that requires paid features and an Airtable account for each user, then there is a cost per user.
Airtable has its own mobile iOS and Android apps. Airtable can also be used as a backend if you are going to write your own app. However, Airtable is not an app authoring platform, per se.