Add employee to specific Team but with restricted folder access

Your correct on the permissions. If you mean folders by channel or on the underlying SharePoint site then the only way you can accomplish this is to make your current members all owners of the team and make the new hire a member. Then you can remove the member group from the folders you don’t want them to access. Keep in mind this will still grant access to all channels chat history.

Alternative solutions could be migrating files out to another SharePoint site collection and attaching these more private locations to a team channel tab or using cloud storage options on the file tab. You can customize the permission on that site and content how you want.

And last you could create new channels that are private that would contain the files that need to stay private but that new person would not have chat access to the channel nor files. So this may not work.