It really depends on your needs, with what you're describing a simple spreadsheet would be all you need.
But other companies need to integrate with certain systems that have authentication where you want to allow/disallow access.
These are jobs for LDAP, SAML, OIDC providers.
I personally use Authentik but there's Authelia and Keycloak as some examples.
But by no means are those as simple as checking a spreadsheet. But you wouldn't need to check a spreadsheet because it's integrated in your login page
It really depends on your needs, with what you're describing a simple spreadsheet would be all you need.
But other companies need to integrate with certain systems that have authentication where you want to allow/disallow access. These are jobs for LDAP, SAML, OIDC providers. I personally use Authentik but there's Authelia and Keycloak as some examples.
But by no means are those as simple as checking a spreadsheet. But you wouldn't need to check a spreadsheet because it's integrated in your login page