Biscuit, dowel or simple braces for making unit top with inset lid and slim sides?
Part of it depends on what material you're using for the top. If it's hardwood or plywood, a good quality wood glue and biscuits / dowels / pocket screws will likely hold it together just fine.

If it's something like MDF or OSB, then I'd want some additional joining mechanism such as dog bone fasteners.

Braces below are fine, as long as they don't interfere with the attachment of the top to the cabinet. Rather than thin braces, though, you could do wide flat ones, and glue, preferably with nails and/or screws. That would give you lots of surface area for the glue to adhere to.

I'm a belt and suspenders kind of guy, I typically glue + screw or glue + pin nail things together.