Is there an example or doc re: how to read a file (image, document, etc.) from an external URL and as part of storing it in a business object (as a base64 property)?
I don’t think that we have a specific document for this but it should be fairly straightforward. Make a call to HttpClient:
const result = await this.httpClient.get('<url>',{responseType: '<response type>'}).toPromise();
The response type defaults to ‘json’ and you can see other response types in the Angular documentation (Making requests • Angular).
If the data is a blob (for images for example), then use the JavaScript FileReader approach to convert it to base 64.