When multiple groups that each apply different permissions to the same folder are assigned to a user, this is known as an "overlap". In Dash, the most generous group permissions will win out.
Let's say you have two groups:
"Marketing contributor" - gives contributor access to just the Marketing folder
"Basic all access" - gives basic access to all folders
A user assigned to "Marketing contributor" and "Basic all access" will therefore have conflicting permissions on the Marketing folder (contributor access and basic access), and so because the most generous permissions win, the user will ultimately have contributor access to the folder.
Similarly, if a user is given "Basic all access" from one folder group, but another folder group gives basic access to only one folder, the user will have basic access to all folders.