Restrict access to a sub folder

Hello - We are just about done with our permissions and folder structure. However, we have one more request. Let's say you have two employees and each employee has access to a root folder. Once in the root folder, there are two more folders that both employees can see. How can I restrict access to folder A to employee B? Employee B still needs to see the folder, however. Just not access it. Same with the other employee and folder. The end-users will not actually be browsing online, just using File Explorer, OneDrive Corp sync.