When I make changes to resource managers, do they impact just me, my whole team, or anyone using Arrow instance?
Resource Managers in Arrow are defined at the global level and are not specific to individual users.
When user creates new resource manager, only he/she will have the permissions to view, edit and use it, making the resource individual to that user. However, as soon as the manager is shared with another user/user group, they are going to be able to see it and (with sufficient permissions) edit it. All changes made to this Resource Manager going forward will impact every user that can use it in a plan.