SSO on Supabase is mainly managed using supabase sso cli commands. There are details in Supabase Documentation.
We have a run book to keep steps and processes documented on Google Drive
These are stored in 1password under Prod Credentials > SSO Attribute Mappings. They are stored on a per-customer basis. This is because each implementation of SAML could be different between companies - even with the same provider.
To use an existing mapping you will want to copy the values into a temporary json file that is used to run supabase sso update. After you are done making changes please remember to update value in 1password. To know more about these please see Supabase Documentation.