I am stuck on creating the roles and which access to provide to make the dashboards visible. Will the dashboards not show if they were made on My Pages?
FYI @pjc95 I moved your post to the Academy & Training forum.
Re your question, a couple points:
Re (1): You are correct. There is no need to select a Landing Dashboard. Dashboards only apply to the Classic UI. See here for more info on Landing Dashboards for the Classic UI: https://help.anaplan.com/ebf9497c-9760-4762-9910-c25712cc2479-Select-the-landing-dashboard-for-a-model-role
Re (2): Model Roles and Selective Access are actually independent of each other.
To allow a user to "[write] only their assigned Product Family, but only view the data for the other three Product Families", you should enable Selective Access on the relevant list and then assign access for one or more users. For the other security configuation, you configure this using module permissions for the role and then assign one or more users to that role.