The error will be
Define Frappe Server URL in Social Login Keys
No need to add social login key on the IDP only add the frappe server url on IDP.
IDP must have OAuth Client Added. keep client secret and client id blank on idp.
On other frappe server which connect to this frappe IDP, add server url as well as client secret and client id from OAuth 2 client added on IDP.
If you are building single signon for your own website. no need to add client id and secret here at all. Only Frappe server url is enough
Yes frappe can act as IDP. As an example Android Authenticator uses Frappe as IDP. There are other apps as well.
Also OAuth Bearer Token can be reused in various ways. e.g. Custom SMS OTP or QR code based 2FA
Android Authenticator:
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