A BACPAC file extends the DACPAC (Data-Tier Application) format to also include data.
The .bacpac file is a zip package containing the database schema and metadata such as logins, as well as batches of BCP-format exports of the table data.
You can inspect a .bacpac file yourself by opening it up in an archive file manager like 7-Zip.
This uses the Open Packaging Conventions format that’s part of the Office Open XML specification. It’s also known as the Packaging API in modern versions of Windows, and exposed in the System.IO.Packaging in .NET.