In more detail
The usual way into Themescape is to download your tabs as CSV files and load them. Google Sheets mode skips the download for data that is safe to share. You paste the sheet's own link into the viewer and press Connect, and your browser fetches the sheet straight from Google. Themescape's servers never see it.
The viewer reads the sheet once, when you press Connect, and reads the Data and Settings tabs only. For it to work, the sheet's sharing is set to Anyone with the link, and anyone with that link can open every tab while it is shared. The Google Sheets mode is opt in, for non-sensitive data only.
In Themescape
Use it for teaching datasets, pilots or public material. Interview transcripts and anything confidential belong in CSV files, which never leave your machine. Once the map is open, set the sheet's General access back to Restricted.