Usually the surround is built, then the insert is lifted into place in the surround and held with screws. It can be unscrewed and lowered. Hopefully there is an access panel on top so the connection between the duct and the insert can be reached.

Take a look at the linked installation instructions for this GE Profile UVC9300SLSS insert. On page 14 you see that the hood is inserted into the cabinet opening and then held in place with screws. The duct is then attached to the hood with duct tape. So if you ever had to replace the hood, you'd have to be able to access the duct to detach it from the old hood and attach it to the new one. Sealing up the cabinet so the connection between the duct and hood was inaccessible would NOT be wise.