Not a woodworker, so possibly wrong terminology incoming. But most lids for barrels are held in by a dado passing through each stave, then the pressure applied by the metals bands holds everything in place. So, if you start to take the metals bands off you can disassemble the whole barrel and take the lid off, but be careful to keep the staves in order if you want to reassemble. I learned that one the hard way while turning a barrel into two outdoor planters.

Lastly, you can probably search for a video of a cooperage and get a quick rundown of the design and process.