how do I roll back to Sonoma from Sequoia?

Backup, make a bootable Sonoma USB installer and boot to it, reformat, install, restore.

Create a bootable installer for macOS - Apple Support

Notice that some libraries like Photos do not work in Sonoma anymore if a later macOS like Sequoia has touched them.

Did this computer original ship, as new, with macOS 15.x Sequoia ?

Of so, not possible to downgrade to anything older than what it shipped with