SCPs define what an account including account root user can do
While account root user still has unrestricted access to an account, SCPs put boundaries on what an account can do & hence restrict account root user indirectly
Service control policies do not grant new permissions to identities. They only restrict permissions given to identities
Service control policies can function either as
Allow List - I.E Deny by default but allow specific access
Deny List - I.E. Allow by default but block specific access. This is the default for SCP.
As Deny List is default i.e. by default SCPs allow full access or in other words by default they have no effect