Hi @Carlo_Burrafato and welcome to the forum!
As @Bastien_Siebman said, custom_fields is a property of the project object and you can retrieve a project’s current custom fields and their values by getting that info for a project.
In terms of knowing when a project custom field changes, there’s no equivalent to the “get stories” endpoint you reference for a task (but be careful about how you use that endpoint, see below).
You will get a project-change event triggered when you change a project’s custom field value, so you can use either Events or Webhooks to be notified of changes in custom fields (you can set a filter on your webhooks so you only get notified about custom fields changing, not other properties of a project).
A word of caution on using the “get stories” endpoint - it’s not meant as a complete audit trail and can have gaps in it; see this forum post for more.
Let us know if you have further questions or need more info on any of the above!