Hello Frank.
Thanks for reaching out. Firstly, you will benefit tremendously by using the ArcGIS Maps SDK for Swift over direct REST requests (unless you're just making very simple calls to the ArcGIS Location Platform places or geocoding service).
We have numerous tutorials at the SDK's homepage: https://developers.arcgis.com/swift/
I suggest you start with this one, which will introduce you to some concepts around API Keys and the SDK in general: https://developers.arcgis.com/swift/maps-2d/tutorials/display-a-map/
But I also suggest you go through the getting started pages at the SDK homepage.
You can also see these blog posts that we wrote around getting started with the SDK, although they will probably be slightly out of date as we just made some major changes to how API Keys are created and managed:
The new ArcGIS Maps SDK for Swift is the modern continuation of the old ArcGIS Runtime SDK for iOS. Learn more here where we first announced the modernization of our SDKs back in April 2022, and here, where we cover the initial release of the new SDKs later that year. With that in mind, we do indeed discourage use of the old Objective-C ArcGIS Runtime SDK for iOS (which provided UIKit UX components), and although it is a long-term support release, it will eventually cease to receive support. Apple's stated path forward is Swift and Swift UI, and this is what we've committed to with the ArcGIS Maps SDK for Swift. However, Apple provide an easy way to use the Swift UI components with UIKit using a UIHostingController (see Apple's doc here and here), and we have customers who are successfully doing this.
As for authentication, you can learn all about using our API Keys and OAuth here: https://developers.arcgis.com/swift/security-and-authentication/
Which one you will use will depend on the type of app you're building and who will be using it. As a general guide, many Esri end users are direct users of the ArcGIS system and have their own logins which enable access to content and services. These customers may well need to use OAuth authentication. But if you are building apps that are to be used by end-users without an ArcGIS login, an API Key is more appropriate.
I recommend you create yourself a free ArcGIS Location Platform account which will allow you to create and use API Keys of your own. There are generous free monthly allowances allowing for development and testing, and which can also be used in production. If your customer has given you an API Key, you can certainly use that and it ensures that any usage beyond the free allowance will be billed to them (or just fail if they haven't set up payment methods), but there are a couple of disadvantages to that that I can think of. Firstly, API Keys are scoped, and if you need to provide access to additional services, the customer who hired you will become a bottleneck. Secondly, you won't have insight into usage levels for the API Keys, nor the ability to create additional ones. However, if the API Key is just going to be used for simple stuff like accessing basemaps and performing geocodes or place searches, you might be able to get away with it. More likely though, using your own account during development and testing will work well, and then the customer can provide an API Key for production.
I hope that helps.
