Majority of Appsheet users are based on Google drive and sheet as default data source.
If your data is sitting outside that, and need to access those data on Google sheet, I recommend combination of Apsheet GET API pluc this google chrome extension, called API Connector.
There is bunch of similar service, like Supermetrix etc, but this extension is simple and easy to handle.
For non-coder, we dont need to type GAS or any other script, but just set up API call Appsheet api to retrieve data.
For development environment and/or just get data from other source to googlesheet, this is quite handy and useful tool, which I personally use quite often.