Custom detections are now the unified experience for creating detections in Microsoft Defender

1. Yes - today you can use custom detections to search in all XDR tables and even join them with data you have in Sentinel - without shipping the XDR data to Sentinel! This was written in the highlights section in the blog (I assume this was not clear?): 

• Cost reduction – Write a detection combining XDR and Sentinel data without extra Sentinel ingestion costs.

2+ 3. Sentinel repositories will indeed support custom detections, that will be deployed to your Defender tenant (and not to azure). We can't commit yet to the specific details as JSON structure and setup process. We will publish detailed documentation when the feature is ready and available to use :)