Cody,
Before we talk about the differences, let's talk about the similarity. Both Quick Report and Survey123 are built with/powered by AppStudio for ArcGIS, and the source code of both apps is available in AppStudio Desktop. They are both used to do data collection to a feature layer.
But there are major differences, Survey123 is a complete product, but the source code is shared in AppStudio Desktop for customers to customize it and Quick Report is a configurable template provided by AppStudio team, the configuration can be done in AppStudio Desktop, the source code is also available. Below is the product description from each website, feel free to look at each website and play around with the app.
Survey123: Survey123 for ArcGIS | Create Smart Surveys & Forms for Data Collection
Part of the Esri Geospatial Cloud, Survey123 for ArcGIS is a complete, form-centric solution for creating, sharing and analyzing surveys. Use it to create smart forms with skip logic, defaults, and support for multiple languages. Collect data via web or mobile devices, even when disconnected from the Internet. Analyze results quickly, and upload data securely for further analysis.
Quick Report template: Configure Quick Report—AppStudio for ArcGIS | Documentation
The Quick Report template is for creating citizen engagement apps that allow users to capture an observation and submit it to an online service.
Can we do a MapViewer app with Survey123 built into it? Technically, yes! The source code for both apps, Map Viewer and Survey123 are available in AppStudio Desktop. You can start with Map Viewer, then customize and add Survey123 functionalities, or you can start with Survey123, then customize and add Map Viewer functionalities.
I hope this is helpful,
Erwin