Hey Freddie - so glad that you've been enjoying Figma Make!  Especially happy to hear that it's fit into your style of work 😁

To address your questions:

Can the public use it?

If you've published your Make file, it can be accessed publicly, if your URL is shared. You can publish your Make file, but then also unpublish it (see here)

Is there any security / How to manage / see who is using it

First, I can assure you that Figma takes data security very seriously. For all customer data, including content in Figma Make, we:

• Encrypt all data at rest and in transit
• Use security measures designed to protect against unauthorized access
• Enforce tailored permissions and user access controls

Figma works with third-party model providers who are not permitted to use data that customers upload or create on the Figma platform for training their own models. Any data stored by these providers is temporary and solely for processing requests and enabling AI features. We also take additional steps to de-identify content and redact sensitive information from both text and images. Control AI features and content training settings

Also, this blog may be helpful: https://www.figma.com/blog/8-ways-to-build-with-figma-make/ 

How big your app can be

I would encourage you to build what you want to build, and then refine to the size that fits best for you 😀 If you run into issues with performance at a certain point, let us know. We can take a look to see what’s going on, and things we may be able to improve.

Hope this helps!

I honestly cant say you enough Thank you enough you have changed my life, How  can i manage a user based app.  Seeing diffrent logins and managing users for example is this possible.