Yes, as long as you are able to share it, they will be able to access it. If you mean accessing folders in a Team "by default", that depends on the privacy setting of the Team, and generally speaking only members can access it.
We assign a channel to each project in our department and each channel has different stakeholders - thus we do not want to share the entire team (and all channels) with all clients. Until Microsoft enables an easy way to add people to single channels..I think using using the Teams doc library permissions may be the solution.. the trick is to first create the Folder/file in the Teams interface, then manage permissions in the Sharepoint interface.
