I would like to inform you that the
SP_Farm is the domain user that you used to configure your SharePoint farm, that must have
DB creator ,
security admin as SQL role on the SQL Server level.
And as you said if you have used it during configuring SharePoint so by default,
- it is a member of farm administrator group that can't be removed from it.
- it is the account that runs SharePoint timer service.
- it is the account that runs the Central Administration application pool
Regarding : at health analyzer you get the
Accounts used by application pools or service identities are in the
local machine Administrators group
this issue occurs because of the farm account is a member of Administrators group on the local computer, meanwhile, it runs Central Administration application pool , SharePoint timer service.
And to avoid this health analyzer warning , you should remove the farm account from Administrators group on the local computer and make sure that any service account or application pool account is not a member of Administrators group on the local computer.
Also, check the details steps at Accounts used by application pools or service identities are in the local machine Administrators group within Health Analyzer Issues in SharePoint Server