I wrote and article about:

Creating unique file permissions with Microsoft Teams

but I thought I’d also do a video:

https://www.youtube.com/watch?v=13BifpwKTt4

as I do get this question a lot about having a different set of file permissions for users inside a Microsoft Team. Best practice is NOT to alter any of the existing permissions that are provisioned by channel creation. Instead, create a separate area, with the permissions you want, and then link that back into your team.

That provides a lot more flexibility and doesn’t ‘break’ any of the standard settings.